Fix for issue where silenced channel will continue to play previous content

[sylverb]

This is fixing a bug where a silenced channel is still playing previous content. This can be heard in Celeste Classic, when reaching the "Old Site" a channel from previous song will continue playing. This is also fixing an incorrect assignment of -1 value in the speed variable which is unsigned, probably not important as the loop on channels is supposed to update this value, it just makes code cleaner.

[jtothebell]

Thank you for this fix, and good catch on the signed variable getting an unsigned value.