Hi all,
Voice memo recording and listening through iMessage is suboptimal and I have found a few ways to work around the major pitfalls including the frustrating feature that cuts off a recording when your screen auto locks. The best option is actually to just switch to WhatsApp because the voice memo sharing features there are superior and will play through auto lock and will continue playing when you switch apps.
For long voice memos:
1) If you press and hold on a memo sent by someone else, it will expand the dialogue box and if you press on the waveform graphic it will start playing the memo in a large dialogue box. From here, download the note to your files and then locate the file and play it - the playing memo will override screen lock from here.
2) When recording, I suggest using the "Voice Memos" app and then sending the recording as a file, rather than sending a voice memo within the iMessage app.