From the experts:
Bix is a cool and jazzy nickname name, thanks to that final x. It's largely associated with the legendary and influential cornet player (and inspiration for the novel Young Man With a Horn), Bix Beiderbecke. He was born Leon Bismark Beiderbecke and his nickname derived from his middle name.
Bix could easily fit in with cousins Jax, Dax, etc.