You can't control which channel the sound takes, it will always be in the sound channel.
You can query the offset of the sound to see if it's through:
local offset = getSoundProperty(id, "offset")
If the offset is -1, the sound is through. Many cards handle up to 256 sources (channels, stereo means 2 channels), some cards even 1024 sources, but other cards can only do 16. You often are better off using not so much audio channels, as it will be hard on the cpu after some channels.