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,
rick2718
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. 
fm muting blocks out weak reception stations...some tuners pick up stations with no antenna, some only with a good antenna...