Cosmetic: Tweak the OSD message for audio delay changes.

Make it consistent with the OSD message for subtitle delay changes.

Fixes #4989.
Grammatical fix: Units should be preceded by a space in English.

Translations completely identical to the English version have been reset to encourage the translators to double-check that the translation was really correct.
Fix: The "Channels" sub-menu was not translated anymore.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.