SCUMM: Use correct sound type for digital sfx.

This makes sure that digital sfx in pre digital iMuse games are using the SFX
sound type rather than the speech sound type. This allows proper volume
control.
SCUMM: Make sure digital SFX are played even when speech is muted.

This makes sure that digital SFX are started even when speech is muted (when
using subtitles only mode). This is, for example, noticable in the Sam&Max CD
intro.
SCUMM: Fix Ctrl+t key code combination to affect speech mode.

Formerly, the key code combination didn't unmute speech at all.
AUDIO: Increment total play time when decoding MP3 data.
AUDIO: Rename MP3Stream's _totalTime to _curTime to better reflect the variable's purpose.
Merge pull request #483 from lordhoto/scumm-sfx-fix

SCUMM: Fix playback of digital sfx when speech is muted.
Merge pull request #485 from Akz-/mp3-seeking-fix

AUDIO: Increment total play time when decoding MP3 data.
Merge https://github.com/scummvm/scummvm