I have experienced this a couple of times from only one contact. We frequently send and receive audio messages to each other and we can't seem to pin point what triggers it but once the troublesome audio message arrives it locks up the whole thread/convo; barely able to scroll, unable to type, incredibly delayed response when pressing and holding any of the messages within that one thread if that even works at all. The audio message in question also reads a 00:00 time, so you know which one it is.
A note that other threads/convos with other contacts seem to work fine.
I have previously been able to fix it by deleting the one audio message, but it is a nightmare to do because the message app locks up for that one convo/thread. I've tried on my iPad too but it locks up the same.
I've found if I manage to scroll a little bit to get the audio message off the screen then I can press and hold, then select the message to delete. Last time I was able to delete the audio message but this time around it's just freezing, I can select it, press the trash-can and then it prompts me with a "this will delete this message" and I can't for the life of me get past that part - it freezes.
The contact can't unsend the audio either because she has her audio messages set to delete after a couple of minutes, whereas I don't.
Nightmare.