Note - WAP and other mobile groups haven't been involved in our activities. Perhaps we should invite them to participate. Look for a host involved with multimedia or mobile. You need to be able to control the speech volume independently to improve the signal to noise ratio. There are a lot of cases where sounds are used to provide information and turning off background sounds prevents access to information.
Implementation sounds like a mixer panel.
Recall that P2 for this checkpoint was in part based on the WCAG assumption that there's a P1 requirement for text equivalents for sampled sounds. Therefore, the information would still be available, even if you turned off background sounds. Just because WCAG 1.0 requires a text equivalent doesn't mean that it will be provided. Current screen reader technology only allows configuration, not dynamic control. Real Audio has a control on their media player.