Voice messages currently unnecessarily waste a lot of data. The size (bitrate) of them increased with some of the latest updates. This results in no benefit but a waste of users data plans. The bitrate should be set to an appropriate value which better fits mono voice messages.
For making a good decission for the bitrate, this may help: https://wiki.xiph.org/Opus_Recommended_Settings
They recomend just 24 kbps for voip or mono audiobooks. (But I would argue that this could be a little low.)
I recommend something between 32 and 64 kbps.
I have done some testing with a good studio microphone and stopped hearing a quality difference at 56 kbps. That doesn't mean Telegram should use 56 kbps, considering that most Android users won't record with a studio mic but the crappy built in mic of their phone, even values below 56 kbps should be fine.
Bitrate testing summary:
32 kbps: medium to good quality, saves users data, but compression may be noticable depending on the mic and scenario
48 kbps: the golden middleground ???
64 kbps: very good quality, upper limit of where a difference could be heard for voice recordings, increased data usage for users
Client version comparision:
the current client uses a bitrate of ~160 kbps (overkill! - considering that xiph recommends only 128 kbps for transparent quality on stereo music, this is absolutely insane for mono voice recordings)
older client versions used ~ 32 kbps
Steps to reproduce
Use latest Android Client
Record a voice message
Notice that it has a big file size & inspect it (have a look at the bitrate)
Device info
OS: Android; multiple devices and vendors tested, problem is device independent;
App version: 13.0
I've been noticing this for months and can confirm that unfortunately it's still an issue in february 2023... voice notes sent by android are huge for no reason, on desktop (and iOS as well I think) the bitrate is 32 kbps.