I use voice messages a lot and sometimes there’s a necessity to postpone them to hear them later. As in those cases you might chat on with the person the voice message becomes “invisible” as it moves out of sight and you maybe don’t know if you heard it already (especially if there are many of them) or you simply forget about it.
In Telegram there’s a really great feature which shows whether a voice message has been started once by the recipient. Some might not know this feature so here’s a screenshot:
This small dot disappears as soon as the message has been started once by the recipient. There’s no way to restore this dot so it helps both parties to easily see if the voice message was listened to already (more precise: started once 
).
And to make it easy to find voice messages which are still “unheard” yet Telegram also has implemented this dot in the “search filter”. It looks like this:
This way you can easily see which voice messages are still pending to be heard which is really helpful if you use this feature a lot. And it makes it possible for the sender to remind you if he sent you a voice message which was not heard yet (already had this situation with Telegram more than once and was thankful this feature exists).
So my request would be to implement a way to be able to see if a voice message was started once by the recipient and to be able to see this status (with a dot or whatever you find more appropriate) in the filter GUI too.
Looking forward to your thoughts on that 