Wrong durations stored for some sounds

[ffont]

Some longish sounds have shorter previews, suggesting that some errors while creating the previews result in shorter files. The problem is that players rely on the stored duration of the sound and not the file's duration, which causes some trouble (e.g. see sounds in this pack https://freesound.org/people/csengeri/packs/19901/). The issue is specially visible in the new UI because the time indicator in the player does not match with the expected sound length.

We should define a way to identify the sounds which have wrong durations, and fix processing code so previews are generated as expected.

[alastair]

Related: #1178