What's this button for?

First try of a new/old Sansui qrx 8001, one pair of speakers, power on, fm, no sound. Volume to 3, tiny hiss, volume to 5, verrry faint station, volume to 9 I can just hear it... wondering if the $2000+ refurb story was bs, trying to figure out what's wrong when - hey, what is this "fm muting" button about?

After I peeled myself off the opposing wall and got it turned down I'm wondering if there may be any significant likelihood of damage,  and/or anything I should watch for or fix?  thanks,
fm muting blocks out weak reception stations...some tuners pick up stations with no antenna, some only with a good antenna...
Sorry to be morbid here, but when I was on Wall Street back in the 80s traders started quipping these were Christa McAuliffe's last words aboard the space shuttle Challenger.  I know, I'm a horrible person.  But this is the first thing that popped into my head when I saw this post.  I do apologize for my political incorrectness and complete lack of sensitivity to legitimate American heroes.