Some libraries are better than others with quality control. I would report the distortion back to the library sales person/liason and let them know their stuff is distorted and it is taking you extra time to fix their mess.
Some music libraries are run by conglomerate businesses who think music is music and they just try to push as much of it out of the door as possible. I have literally had people flat out tell me clients cannot hear the difference and so why spend the time making things pristine or good sounding. I find that hard to believe and this post is a testament to the fact that clients can and do hear the difference.
As to Brent's comment about why bother with mastering... There is actually a two main reasons why most libraries master their music.
The first reason being is that generally the music is usually not initially "picked", while listening on the dub stage mixed into the movie [DISCLAIMER: Every show has a different workflow, so some might but that is not the norm] . The music is picked in some sort of search engine, usually with various different libraries all combined and jumbled together. You do a search for "classic 40's big band with vocal" and a list of tracks come up from whatever libraries you are signed up with. You go down the list listening to the tracks and marking certain ones you think might work.
In that list... files are not automatically volume adjusted by the player. So a track that is 10dB or 20dB quieter than another will sound (because of the fletcher munson curve) wimpier and of less fidelity and a client unknowingly will be less likely to choose it.
The Second reason library music is mastered is to try and get the library track as close as possible in sound to the original music used in the temp track. Lots of editors and directors use temp tracks from their own CD collections. Whether its music off a Dark Night or Bourne soundtrack CD or a track from a Katy Perry or Adele CD, the music being used in the temp (unless its original score written by the show's composer) has been professionally mastered.
So... after a list of tracks have been selected that could possibly work... then the editor/director/music sup/whoever has to try to find the ONE track to go with. And that selection boils down to which one most closely matches the temp in intensity, feeling, mood, impact, etc. And again, the mastering can aid in that.
I always say this. Music libraries do not compete against each other for placements. They compete against the temp track.
Audio Engineer - Producer - Composer http://www.linkedin.com/pub/derek-jones/8/986/9b9 http://www.myspace.com/daogkilla "We were working on Raiders [of the Lost Ark]. He [Ben Burt] told me that the sound source for opening the lid of the ark in the last reel was within 20'. I couldn't figure it out. It turned out to be lifting the back off the toilet above the water chamber, and slowing it down." -Tomlinson Holman