My speculation is that the PCE's sound, without tweaking, tends to resemble lesser 8-bit PSG tones. Case in point, the soundtrack for Eternal City totally sounds like it could have been pulled off by the Famicom.
However, I find the PCE typically has more FM-like sounds & while they're not quite to the level of actual FM, it still sounds unique and interesting. Go listen to the soundtrack of Shubibinman 2 & you'll see what I mean.
Like Esteban said, it's all about how ambitious the music programmer is wanting to get with the hardware, and really nothing to do with the hardware itself. Even the Famicom can sing thanks to its versatility with extra chips and stuff. The master system PSG I think pretty much sucks though...it's not impossible to listen to like Atari chip sound, but still quite basic.