Well, it must be something with that sound, maybe it's still playing. But I thought there was a limit for how long MP! sounds could be... hmmm... Well, normally if a sound is playing, the ones that are received
while the last one is still playing, will not be played automaticly. As you wrote, I suppose your settings are to automaticly play incoming sounds, which then usually would be that the sound is still playing.
PS: You can click stop, or just don't use that specific sound.