You should be able to upload MP3 files, but it has to be smaller than the maximum upload size. To increase the maxium upload file, if you are using the filesystem to store files (default) then you need to set a higher value to some PHP config options. For example, to set a 10 MB file limit:
max_execution_time = 300
post_max_size = 10M
upload_max_filesize = 10M
This can be done in a php.ini file in the root of OpenGoo. The max_execution_time determines how long a PHP script will run. The higher the better for safe uploads, but your hosting usually imposes a limit to this value. The other two are self explanatory.
If you are using the databse to store files you need to additionally set a MySQL config option called 'max_allowed_packet' to something greater than 10 MB.