If the error is right at the start where the upload never gets going, then there is a configuration error somewhere blocking the mp3. If it is after the file has sent half of itself, then the error is with the machine, full traffic, or a bad connection to the website.
some areas to check assuming it works for small files.
the form (max file size too small)
php.ini (max upload size, max post size and a few others-- you should not have a problem as it is usually 8m -- to check use a clean php.ini and search and replace the 8m with 120m as well as turn upload on)
timeout (in your server, you should override this in your script by setting the timeout)
cgi time out (if iis, and it dies after 3 minutes)(this is hard to find in a subtabbed area under the main subtabbed area, if IIS you should find my previous post where i went into more detail. IIS is not very user friendly at all)