Streamed music sounds awful in my system and I don't know why ....

Get the PC out and put ANY streamer in.  I have a Node N130 and it’s much better sounding than my PC. I’m sure i do better, but this is where I started.

Oh, maybe I missed it, but what streaming service are you using? Good or bad, they all have a different sound.

All the best.

If his PC is that bad, he needs a serious upgrade. Either the router is somehow failing at that point and allowing some sort of back feed, which I don't see possible, or the PC itself is a piece 'o' CHIT. I am constantly amazed at how people are willing to pay tens and hundreds of thousands of $ for audio equipment and then put in some sort of junker PC into the mix. I don't even have my case sides installed and I get '0' interference from the PC. A Simple test of TURNING OFF the PC might prove if that is the source of the problem. I can stream multiple sources and I get as good of sound quality, from the PC and more convenient than even my dedicated Streamer. But it is what I work with 16 hours a day also. And convenience is not sound quality. I would start replacing Less expensive and easier-to-try components like that MINIMAL CAT6 cable. An acceptable CAT 7 or 8 Cable can' be acquired for chump change. Also if it is the cable, try replacing it with ANYTHING. Even lesser quality should show a marked difference if it is the cable causing the problem.
Then find a friend that might be willing to let you try a piece or two of their equipment in your system or to put some of your pieces in their system. Even if theirs is of a lower quality.

Something else I would caution people on. Reports like this one from ZDNet magazine.

How to convert your home's old TV cabling into powerful Ethernet lines | ZDNET
www.zdnet.com/home-and-office/networking/how-to-convert-your-homes-old-tv-cabling-into-useful-ethernet-lines/?ftag=TRE-03-10aaa6b&bhid=%7B%24external_id%7D&mid=%7B%24MESSAGE_ID%7D&cid=%7B%24contact_id%7D

These peopl eare not audiophiles and yet when reading some of the reports in these forums I wonder what people ARE following.

You can modify the pc diy route as some above have mentioned, you can also go lower, medium grade off the shelf streamer, and then there are top of line streamers. My choice is top of line streamer budget allowing. Streams should be virtually indistinguishable from cd over transport or cd rips, you'll know you've arrived when this is your experience.

a few points.  While i agree that the problems re likely with WMP and the PC, i want to re-iterate where the problems likely lie. Not with the simple fact that you are using a PC -- but with two things:

1. The configuration settings, some of which may be very hard to optimize

2. With the electrical (ground) noise

Both of these can be ficed, but most wont have either the expertise or the inclination to do so. On a mac you can use bitperfect and the configuration issues are solved.  Gond isolation is more of a problem, although on battery power my macbookpro was more or less fine.

Many have suggested Roon.  I love roon, and also hate roon. Note that you will still have the ground noise problem if you connect your roon PC directly to your DAC.

Roon has also just made major changes that i find a real problem. They discontinued support for most operating systems aside form the very atest - making most of my units obsolete even as casual remotes.  They also have made it so that you cannot play your own local library unless you have an active broadband internet connection.  You heard me right.  So yea, its great, but its also nto the most customer friendly company on earth. This is all very new BTW.  Beware.

I do think that software solutions liek Roon or audirvahna are fare more modern and long-term than some costly streamer-in-a box (which is almost certainly a raspberry Pi computer inside a fancy box with a good power supply and isolation).