Make sure you have some back up players. I'd recommend Audion & ToolPlayer. The latter is just a very simple player but has an amazing wide range of audio formats it can play. Audion has not been developed for a few years, but is still a good player all the same. It can play mp3pro which iTunes cannot, likewise for Ogg Vorbis files. And it can open playlists & create its own. Alternate Audio Players & support plug-ins

Yes my guess is those files you downloaded are unfortunately corrupt in some way. Again, check file sizes. For AIFF, you know that 1 minute is around 10 MB in size, so if it shows as 4 minutes & is less than 35 MB then you know it's probably fake or corrupt.

Bit rates, yes here's that post http://www.gnutellaforums.com/222466-post13.html

Oh .. mp3's that do not show any information when you hover your cursor over top in the search results are most likely fake. AIFF unfortunately rarely have such information tags so it's not easy to know. If you know the length of the song before hand, then that may help for AIFF files. I guess one second of an AIFF file runs for around 160 - 170 KB. Hope you're good at maths. lol (size of AIFF may vary very slightly depending on OS & OS version, etc.)