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,