Need a few specifics here -
Are you buying CD's or downloading music form legit siutes or downloading form music sharing sites?
This can matter a great deal. The CD has the best quality. Downloads are often MP3 files and especially when listening on headphones you CAN hear a difference: MP3 files tend to have a harsher sound.
Also what specific tracks in the shows you mentioned are problematic and what are you hearing? Distortion? Reverb? Indistinct lyrics?
Millions have purchased RENT and although there have been some complaints that the performances seem underpowered, I haven't heard many (any?) about the overall sound quality.
I personally prefer the sound of the older cast albums where cast and singers were all together in one large studio to record the show - The recordings sound more like the shows sounded in theatre. The modern technique of isolating the singers and having them sing so close to the mics, and heavily editing the performances creates an almost sterile sounding recording., Yes everything OS correct and perfect but somehow the "spark" of a live performance is often missing. Even so, the overall sound quality of most modern cast recordings is tremendously life-like.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com