How to disperse avail. amperage?

Don't add up amps, it's not the way its done. I count 5 circuits for audio, one for lights and one for AC. Usually you assign 360 VA (watts) to each receptacle plus air conditioning plus lights. The AC (11 amps at 240 volts) is 2,640 VA. The lights are 1,500 watts at most. That adds up to 5x360 + 2,660 + 1500 VA = 6,000 VA. That's only 25 amps because you're bringing in 240 Volts to the subpanel. You're barely going to draw half that under normal circumstances.

Even if you maxed out all circuits at 1,800 VA per dedicated line, that's still (5x1800+2640+1500)/240 or 55 amps at the ridiculous most you can possibly plug into.

Use a 100 amp subpanel with a 60-amp main breaker. At the main panel in the house, pop in a 2-pole 60-amp breaker and run the appropriate wire size (use 100-amp #2 wire @ 60 C) with the appropriate feeders, mind your grounding, and you'll be fine.

Inside, use 20-amp breakers for the receptacle circuits, 15-amp for the lighting circuit and check the manufacturers literature for the maximum breaker size for the AC unit (important at 240 volts).

Finally, put trust only in your electrician. Good luck and have fun towards your new project.

BTW - you'll have enough power this way for a well-stocked woodworking shop. Would add a ton of value to your house.

Lissnr,
You did not mention how far from the house the garage is.
Distance from the house main electrical panel, to the new audio room sub panel? You may need to take into account voltage drop.

You stated the new sub panel feeder will be buried 18" below grade.
Direct burial cable or PVC conduit with single conductors?

I would recommend PVC conduit over direct burial.

Copper or aluminum feeder conductors?
Copper will cost you an arm and a leg.....

And I hope you are not thinking of an isolated dedicated equipment ground rod.

The equipment grounding conductor shall occupy the same raceway or cable as the feeder conductors and shall terminate in the same panel as the feeder conductors, NEC.

I'm expecting the main breaker on the house side of it to probably only be 100 amps? as well as the main breaker on the sub panel to only be 100 amps too... mostly because

First off you do not need a main in the sub panel, that is unless your local code requires it. If your local code does not require a main then all you need is a main lug only panel.

The sub panel and sub panel feeder will be protected by the breaker at the main electrical panel for the new feeder.

As for the sub panel what brand are you looking at?
I would use a panel with copper bus. If you have the money I would go with a Square D NQOB panel board. The bus is copper and the breakers are bolt on.

I'm expecting the main breaker on the house side of it to probably only be 100 amps? as well as the main breaker on the sub panel to only be 100 amps too... mostly because the 100 amp breaker is 1/3 the $300 cost of the 125 and I can't imagine I'd be drawing that much current all at one time?

I do not believe you could install a 125 amp breaker in the main 200 amp electrical panel anyway. It might fit but the panel is not approved for that large of a breaker.
Curious, did the electrician say he could?

Thank you both for your replies, between my electrician's visit today (I picked his brain for well over an hour) and the info. I'm getting from both of you, I'm learning a lot more than I've ever been aware. The biggest lesson I now understand is that a subpanel rated at a certain amperage (ie 125A) can have many more breakers on it, simply to dedicate the individual items, than the total of all the breaker amp ratings combined. This opens up considerably more flexibility regarding lights, A/C, dedicated audio lines, etc.
Gs5556: I will do the breaker designations as you described, namely 5x20A dedicated audio, [probably] a 20A for the A/c unit, and a 15A for the lighting. I also mentioned to my electrician that I was considering the possibility of adding a 1/2 sized refrigerator (like the ones in hotel rooms/college dorms) and he said I should put another 20A breaker dedicated for it alone... (I thought I'd mix it on the A/C line and he said the A/C should have its own circuit...) If I took this whole concept to the max, so to speak, does that mean I could conceivably add one more 20A breaker for "future potential" home theater items such as a surround sound receiver/amps, projector or big screen LCD TV, etc??? This is the time to ask right? No, I can't think of anything else...That would make a total of :
1x15A lights
1x20A A.C
5x20A dedicated audios
1x20A refrigerator
1x20A home theater
and thanks again for the kind words : I hope it does add value to the house!
Jea48: the house panel to garage distance is only about 40' in a straight line BUT I don't have the option of digging that trench for the cable straight due to trees and a patio in the way. My electrician recommended that I run the main (indoor type) wire from the house panel along inside my patio soffit for a distance of 65' until I get to the corner of the house where I can THEN run a junction box of sorts to another 45' of the heavy duty outdoor direct burial cable in an unobstructed trench to the sub-panel placement point. I am not looking forward to delving into a perfectly neat soffit to do this but I think I'm out of options. All cable will be copper.For a few $$$ more you'd recommend housing the direct burial cable in PVC? OK, can you estimate what I.D. PVC I will need? I don't know what size breaker he said he'd mount at the house panel... it won't take a 100A? He kept trying to talk me into exactly what Gs5556 advised: a 2 pole 60 amp breaker... maybe I should listen to the experts instead of carrying on like I think I know what I need....(but if I add the home theater and small refrig options as above is the 60A breaker still OK?)
As for grounding, I asked him if I could run a grounding bar (or two) from the grounding point on the sub panel (yes, I am looking at the Square D QO series... I think it's the
QO11224l125G which I initially picked out on-line). He said I could run 1 or 2 if I wanted, spaced 8' apart both on the same bare copper cable (did he say 8G or 6G???-I'll ask again...). I did not mention to him that I wanted to run Hubbel 5362 I/G's and run each ground wire individually back (to the sub-panel I assume??) using individual 10G solid copper wire for each and every terminal [and that's what's going to the grounding rods] [from the sub-panel]. I know it's labor intensive but from what I read on these forums it's supposed to be worth it and I was going to do all that wiring myself, then the entire job would be inspected by the town inspector (who my electrician notifies for me once he's checked all my work first). This is what I'm hoping... maybe he will let me run the Romex through the studs but won't let me make the actual connections? I'll find out later... right now it's "part 1: get the power to a sub-panel". Input/comments/help/corrections = highly encouraged! and thanks already for all your help fellas. Best regards...