Add audio seeking within the downloaded audio data

[Br0000k]

Lagrange doesn't have a slide (or even shortcuts) for audio playback control. Currently, you can only rewind to the very beginning.

[skyjake]

This is one of those "high cost, low benefit" kind of features. It is on my longer-term to-do list, though. The UI implementation is pretty easy, but each audio decoder (WAV, MP3, Vorbis, etc.) needs to handle seeking in its own way.

The current audio playback feature in Lagrange should be considered a preview of the audio content — much like the inline image viewer, the inline audio player is very limited in what it can do. For a better viewing/listening experience, the file should be saved to downloads and viewed with another app. (And that could also be improved so that launching such external viewers is easier.)