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.
Not sure what could be wrong exactly; I have tried to play the file in WMP, and it worked fine.
You mention that the MP3s play fine through foobar2k - is it possible that foobar2k is the cause of the issue?
I have my doubts; I've used foobar2000 as my music player for years, and it has not caused this issue in the past.
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?
I suppose I can give that a try. I'm running Microsoft Security Essentials as my virusscanner, which also doesn't produce this issue on my main PC, as far as I know.
Be sure to test this in a clean install of mIRC by running the mIRC installer and choosing the "portable" install option.
I already have both the current and the beta installed clean, as portable, so far, both of them have reproduced the same issue.
I'll give it a shot to play the MP3 again after a clean reboot.
EDIT: Just tried it again after rebooting, virusscan off and no programs running (apart from the usual system and driver related services), but the problem persists.
Very very strange.
