Corrupt files can be due to various reasons.
1. The file is corrupt at it's source.
2. There's been data lossed during transfer so LW will attempt to redownld the missing bits.
3. The hashtree has changed from the moment you started to the moment you finished downlding. (Hash Tree explained: 1. No. 1, & No.2)
4. You have firewall or anti-virus programs affecting the downlds.
5. For audio files see T3 Spammers; mp3 Bit-Rates Versus File Sizes