I've never seen the REMI file extension before. It's a long shot, but perhaps you meant MIDI?
In that case, something like
http://solmire.com/ is probably a quick solution, but a
DAW like your GarageBand will probably give you better control over the actual output sound.
I have FL Studio, so just send it over if you want me to convert it.
If you wanted to convert a GarageBand project (you mentioned .band in the thread title), it should be
pretty straightforward.