[NOSQUASH] Sounds: Fix some fade-related bugs

[Desour]

Fixes two bugs introduced in #13633:

• First is a missing conversion from milliseconds to seconds.
• Second is that the sound manager used to not be stepped while paused, and hence did not fade sounds while paused. But now the sound manager thread always steps. I forgot to handle this.

To do

This PR is a Ready for Review.

How to test

• Place a jukebox in devtest, and play some long sound, e.g. music.
• Set the fade step on the right to 0.1, and target gain to 0.
• For first commit: Press fade. Notice how it is by far too fast in master.
• For second commit: Press fade. Pause the game (via esc). Wait a moment. Unpause, and listen if the sound is still there.