So I was engaged in a text message exchange with a friend yesterday, and along with one of his messages came a strange file. The filename was displayed as "346409300...60457.aac 00:00". It had a 'play' icon next it (the usual right-pointed triangle). I gather that .aac is an audio file format, and 00:00 is presumably the length of whatever the audio is. I asked my friend about it and he had no idea what I was talking about. He didn't send it.

So, (a) How did it get attached to his message? and (b) is it something I should be concerned about?

If he has an iPhone he probably held down the audio message key and sent it accidentally.

^^^ that's my guess too.

In retrospect that does seem like the most likely possibility. Thanks.