A few useful theme music resources: Xylot Themes, a database that lists every known theme for a ton of wrestlers. No downloads though. Fire Pro Archive, a YouTube channel with several themes I haven't been able to find anywhere else. I assume it's someone from the community who maintains this, so thank you whoever you are. Dramatic DDT, a blog that lists theme names for a lot of DDT, BASARA, Tokyo Joshi Pro & Ganbare wrestlers. A lot of them aren't on Xylot Themes. CAGEMATCH also has themes listed for a few wrestlers who aren't on Xylot Themes.
If anyone is interested in themes with "arena effects," I have 1000+ I could eventually share. Currently they're all one minute (longer if they have a long intro, like "Final Countdown") MP3s, but when I find the motivation I'll probably cut most of them down to 40 second OGG files like Phil mentioned. I try my best to normalize the volume, but with so many different song qualities and genres (hip hop is "quieter" than most hard rock, for example) it can be tough.
I just stumbled upon this thread by casuality and thought in helping you out. I keep up as of April 2016 a pro wrestling entrance music project and I decided to log in, even though I haven't played Fire Pro for years (last one I played was Returns). The downsides, though, is that I only collect information, not files. Add that, I only cover North America and Europe (and South Africa if you count that promotion listed). However, to fill that void of not having Japan listed, here's two sites. The first one covers mostly male wrestlers and this one cover strictly Joshi. Plus, I have to mention that despite not collecting files, I record theme songs that were edited or have an intro added, so if anyone wants, you can ask me for a specific song and I'll give you the recording so you can edit it properly.
I know this isn't much help but I hope this would be useful.
After finally getting music to work on ps4 (had to try a different flash drive apparently), I notice a lot of the previous links here are no more. Curious if anyone still updates their themes collections? Either way I'm just loving finally being able to have custom music.