I don't know a lot about the technologies you are using--I've used ogg & jorbis, and decided they were not for me. But they work for most people. However, I know a couple things about sound programming in general and maybe the following will help.
When you say crackle, does it seem to be related to a lot of starting and stopping, or are harsh sounds that are not on the originals playing?
If the first case, the code that is playing back probably is having trouble keeping current. Enlarging the buffers would help, but would also add to the latency. Usually with sound tracks (as opposed to cues for game play) latency is not such a big deal.
In the second case, the volumes from the two tracks could be overpowering the playback code. In this case, you would have to come up with a way to play back the cues at a lower volume. One possibility is to rerecord the cues at lower volumes before saving them as oggs. Another is to programmatically lower the playback volume and see if that helps.
Why two at once? This is unusual, yes? Ogg is computationally expensive, and you are doubling the cpu cost. Is this a situation where the two files can be remixed together outside the program into a new, single cue? Audacity is great for that sort of thing, and free. (Just make sure you don't download a fake version.)
Well, my idea was to have ambience playing along with the music. However, I can see the possibility of fusing the ambience and the fighting theme files and then making a mixer of some kind to just turn up the fighting channel when I'm in combat. However, I wouldn't even know where to start on such a feature in OpenAL.
The crackles happen in a pattern, as in there's like a rhythm to how often it happens. I think it may be the first case. Right now, the ByteBuffer that streams them is allocated to be 4096 * 8, but do you think I should increase it? If so, how much?
Right now the streams are updated in their own thread separate from the game, and I have it sleep for 1000/30 milliseconds to try and keep the engine from stressing the computer too much.
I could also try lowering the volume. Isn't there a command in OpenAL for this? I think it was changing the Sources gain, correct?
Thank you very much for your input.