What I think you should do for diagnostics here, is to temporarily route the level output of the RF module to the radio in place of the MPX output, thus bypassing the part of the RF module which usually goes bad.

If you can hear (tinny) proper sound instead of just noise with that, then we know it's the usual component failure inside the RF module.

I use a small portable amplifier for that here, just touching the probe (amp input) to the level line, and then to the mpx line, and then back again.. and so on, to compare the two outputs.

-ml