Im double posting, because this is going to be long.
There are 2 types of Beatmania Sims out there. Dunno what they are called, but one uses .mp3 backround music, and you hit the keys to it. There is a chance that it could be improperly synced. The other type uses .wav keysounds. These are 90% synched properly, but harder to find. Also, some sims can use both. I used 2 sims before getting the real thing. Rhythem It! was the first, and it sucks. I also used BM98. That one worked very well. Apparantly, theres a work in progress sim called Be-Pachi Music!! I have never used it, but heres some very good links. Sadly, I think you have to register, but I would do that anyway, because its a very good site, and you can get .BM and .BMS files (The song files for your Beatmania Sim) on the site.
www.bemanistyle.com
Heres the topic about all of the new sims:
http://www.bemanistyle.com/forum/f94/where-find-beatmania-simulators-268/
You can find the link to the Be-Pachi Music!! topic there.
In the forum, under Beatmania/IIDX Simulator area, you can find topics and stickys about downloading .BM files. Have fun!