I'd normally say intercom squelch, but you say you've already tried this. The radio should still work with the intercom switched off, depending on how your aircraft is wired up. It can seem a little strange at first vocalising your checks without hearing it in your headset but the radio 'sidetone' should enable you to hear yourself while transmitting.
Otherwise seems to be a fault with the intercom. Try borrowing another headset to see if it's the same.
If this isn't your own C152, report it to the owner and get them to fix it!
TOO