As far as I've been able to "see", YSF does always show low audio level when received on DMR.
I guess it has some kind of link to the need to use YSF on DN modulation when using multimode nodes.
Anyway, in the same line if thinking it does not make make sense when they use their YSF only nodes (so they can use DW and theorically improved audio response) and I have the same results on DMR side.
I agree about comments on low audio level.
It looks like a "brand mark" on YSF.
Anyway it's easy to fix:
Tell the colleague on YSF to SPEAK UP.