Winamp 5.34a Security Patch includes a patched version of in_mp4.dll (v1.22) which fixes the MP4 file parsing buffer overflow vulnerability issue. The vulnerability is caused due to an error within the handling of MP4 files and can be exploited to cause memory corruption via a specially crafted MP4 file. Successful exploitation allows execution of arbitrary code.
Note: if you didn"t install support for MP4/M4A in the first place (WinampPluginsin_mp4.dll), then you don"t need the patch.