Google shows over 1/2 million results for "music master" and over 8 million for music master (without quotes).
In the mobile entertainment industry, it's super common. It's a descriptor, a handle, a slogan... It describes what they do and how well they do it. It's also used in a variety of ways to clearly tell public what they do, for example: a DJ might provide wedding music and master of ceremony services. Or as an adjective, as it's most often used a DJ may be considered a music master. Sort of like mobsters who loan money are commonly referred to as loan sharks, etc... It's just a common phrase.

