It really depends what you want/need. You can do quite a lot with general midi, which means all you need is any sequencing software and a good imagination. You could then dump those to a wave file, and run it through any decent wave editor, like cool or wavelab, and spice it up with some FX, etc. If you want the sound of a semi smashing into a tree, that would be harder to do with the above method, hence it depending on what you want.
hth,
m
The sounds I make come from midi software called CAKEWALK, (my uncle gave it to me, I do not know if it is free) and I use a proram that comes with window 98 se called Sound Recorder to make wav files. It may not look like much, but I think it is powerful enough (for now).
Any way, take a look here: http://www.saber-x.com/spacem.html
Tha BGmusic was made with cakewalk, and the explosions, missle fire, and regular file was made by my mouth with some manipulation in Sound Recorder...
Cakewalk and Cubase are the two biggies in sequencers. I've used Cakewalk since version 3 (worked on windows 3.1;) Downside, they are both expensive. You can do sequencing with java, but I've never done it. But, there are a ton of free sequencers, none of which I've tried, available. I just searched for 'free sequencer software' on google, and this one looks interesting: www.jazzware.com
good luck,
m