It doesnt have to be 320kbs I think the actual limit for mirc (since it uses mci) is 190kbs. Anything above that will produce the errors. (Note it may not be 190kbs for the limit but its near there)
Ive had 2 problems (as with anyone else playing mp3's) It either gives the "cannot find file blah" error or it doesnt play at all. There is no way to correct this and there is no way to halt the error message.
Ive made mention of the try catch onerr methods to be implemented but who knows.