Perhaps its related to the variable doppler_factor? It sounds like that anyway. If that isn't it, it could perhaps be that Shade-c has a similar kind of variable that is experimental or such (would have to dig in the scripts though).
-edit, or it might be related to physx or such in the sense that the xyz coordinates that ent_playsound uses it perhaps not updated automatically. Try updating the xyz values manually too.
I've tried to change doppler_factor and look for it on shade-c files but no luck.
xyz coordinates shouldn't be a problem I think, as I'm attaching the camera to the entity using them and that works fine.
