Some people want to record their Plus! and WLM sounds too, so what about those? (I know you're going to suggest to make it optional though, but still
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All in all, people should learn how to use their computer and learn that for each task there is a program and/or method.
What I mean is that Plus! shouldn't turn into a sound control center, or blogging tool (people have suggested things regarding this in the past), or full blown scheduler/alarm system.
Plus! is an addon for an instant messenger. If you want to create great sounds and effect and such stuff, you should use a dedicated tool. This is the same reason why Plus! only has a very limited trimming function for the sounds. If you want to create decently trimmed sounds, use a wave editor.
It is better to teach people how to do it in the proper way than taking them by hand and treating them as dumbo's. Because in that way, they always will be dumbo's and will never learn to use a computer (and then you get suggestions like "can't Plus! include a picture editor for my display pictures" -also something requested in the past-).
So if someone wants to do what you want, tell them to turn of WLM's and Plus!' sounds. In that way they have learned something new (and something easy), and in the process probably discover other new things....
Note: if it would have been a hell of a job to disable sounds and if it would require editing your registry and all, I would have agreed with the suggestion. But it is dead-easy to do it, so I really see no point to add something which probably will only be used extremely occasionally by only a very select few.
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