Q: Macbook Pro Storage -- Audio

Isn't it a backup of iPhone, iPad and iPod?

See at:

~/Library/Application Support/MobileSync/Backup

That display that you are referring to cannot be relied upon to be accurate due to a big in the program that generates it.  Sometimes, but not always, a Spotlight reindex will correct it:

https://support.apple.com/en-us/HT201716

For accurate information, download from the Internet OmniDiskSweeper and Grand Perspective (both free) and open them. They will show all of your files and the respective sizes.

https://www.omnigroup.com/more

http://grandperspectiv.sourceforge.net/

Ciao.