[Feature]: UX: Allow to play Voice Messages using the silent phone speakers

[p42ity]

Precondition

• [x] I understood that this template is only for feature requests and not for bug reports

Cross-checked related issues?

• [x] I have cross-checked this overview https://github.com/monal-im/Monal/issues/322 as well as filtered for related labels https://github.com/monal-im/Monal/labels

Describe your feature

A user would expect that a voice message (which he/she listens to) switches to a silent mode when the phone is placed next to his/her ear (proximity sensor). Currently, it is not possible to play voice messages using the normal silent phone speaker (which can be heard when you place the phone next to your ear).

[tmolitor-stud-tu]

Would be nice, I accept pull requests