SCUMM: Fix playback of digital sfx when speech is muted.

As the title suggests, this makes sure that digital sfx are even played when the speech is muted. It should only affect pre-digital iMuse games because the latters seems to handle starting a sound differently and I didn't adapt that code path.

This will also enable proper volume control for digital SFX.

It also unbreaks the Ctrl+t key combination. Formerly, when speech was muted, it never allowed to unmute speech.

I only tested this very briefly with the Sam&Max CD intro and one scene in game (at the golf pool). Now, even when I use subtitles only mode the digital SFX are correctly played. If someone with knowledge about the SCUMM engine could look over the changes, that would be much appreciated.