If you do not already have both a multi tool and a good mini flashlight, both would be a great investment. I myself carry a gerber proscout multi tool and a mini mag with and LED conversion in it. I do not, however, wish to turn this into a what is better post about lights and multitools (it has been done). I just think both are great tools for any tech.
Since you are into sound, perhaps a cable tester would be nice to consider. I have the RatPack
XLR tester and have always found it useful for trouble shooting. The nice thing about it is that it fits into a pocket, or an extra pouch on your belt pack.
~Dave