How about looking at this from the other direction.You're an employer reading your CV: Music degree, fired from last job, previous experience presumably limited to working for a sports shop. Why would you employ you if you were an employer?
- Regardless of the type of employment, being fired looks bad - probably moreso if it was from a temporary, 'trivial' or unskilled job where it may seem to the person reading that you got fired in a job which wasn't complex or demanding anyway.
- Music degrees are nice, but like a hell of a lot of degrees today, don't really mean anything at all to an employer - especially if it's for a job that involves anything that's not directly music-related. And administration that happens to be in a music shop is not directly music-related.
- Have you got other administration-related jobs on your CV? As in, invoicing, ordering, formal correspondence, rota-management, etc? If not, chances are that another applicant has, and looks better.
And even then, there may well have been anywhere from 1-20 other applicants, one, some or all of whom might have had better CVs than you, or interviewed better, or both. It's not taking the piss; they don't have some responsibility to employ you just because you applied with an acceptable CV. Obviously it's very frustrating from your point of view, but if you look at the matter as being some great injustice or 'taking the piss', you're not going to get anywhere. You're not entitled to a job with them.