mIRC plays MP3 files through the Windows API, so it depends on Windows to play the file. The most common reason for this type of issue is that there is something wrong with the Windows Media Player installation. You mention that the MP3s play fine through foobar2k - is it possible that foobar2k is the cause of the issue? If you shut down as many applications (including anti-virus software) as you can and then try to play the MP3 again through mIRC, does that make a difference? Be sure to test this in a clean install of mIRC by running the mIRC installer and choosing the "portable" install option.
