The WordPress upload limit is NOT set or controlled by the Music Maker theme, but by your website host.
To increase your file upload limit you will need to contact your website host and ask them for help on increasing your WordPress upload limit to the highest they allow.
Since all website host are different, there is no universal way to increase your upload limit.
Some people prefer to upload to their server through FTP software.
Also check out Can I Upload Multiple Beats at Once?