You can also just install a pre-patched version of Winamp 5.666 (from
http://getwinamp.com/#download) that includes the in_mp3 fix along with another plug-in crash fix as well as stripping out the things that just don't work any more.
That's a lot simpler than having to mess around with multiple installers and as the Winamp developer responsible for that last release it's a safe option to go with.
LG C7