Recently updated to .4130, but I think I recall it happening with the previous public preview. in_mp3.dll doesn't load properly (in input plugins, it shows as "Not Loaded"). It didn't seem to affect me, so I ignored it, until a recently downloaded album just wouldn't add to my library or play when I tried to manually play - it played fine in other media players (VLC, even WMP), so I gathered it was a problem with the input plugin.
I grabbed the in_mp3.dll from the Winamp folder and pasted it into the WACUP folder (the file sizes differed quite significantly, being about 2XXkB vs 1XXkB respectively), and the files imported and played without any issue.
Oddly, other mp3 files played fine, so I'm not sure why specifically these didn't. Any ideas?