some audio messages don't have a length and can't be seeked

### Your use case

#### What would you like to do?

Be able to see the length (duration) of an audio message, and be able to seek in all the audio messages I receive.

#### Why would you like to do it?

- To know how long an audio message is, before listening to it.
- To seek backwards and forwards in the audio message, if i want to re-listen to a specific part / skip a part.
- Because ElementX for iOS does it correctly 😎

#### How would you like to achieve it?

If the audio message's length/waveform is missing from the media, compute it locally (maybe using an audio processing lib).

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

### Are you willing to provide a PR?

No