The TTS needs better spam protection. For example, someone could send a short repeating line of ... and the TTS could take an annoying minute or more to read it all, as has been demonstrated to me on two separate occasions. The existing length threshold is by far not enough to stop that kind of spam while still being usable.
If possible, the TTS should be set to stop after so many seconds of reading any one line, or be able to detect an overdose of punctuation characters and not read it, or at the very least have a shortcut key to stop the TTS output.
The other more common kind of spam, multiple text messages, could be another problem for TTS as it falls behind. For that, it should be able to skip lines or stop altogether when it falls too far behind.
As is common for most chat spam prevention, Mumble (or Murmur) at the least should detect when one user sends a set number of messages within a set amount of time and block messages from that user for a set amount of time.
An automatic temporary ban could also be useful for repeat offenders, the server would only need to remember when the last few offenses were and ban if there were too many in a short period.
I don't think text spam is really a problem in Mumble just yet, but seeing how popular Mumble is becoming and how strange some of the users are already, it's inevitable in the not-too-distant future and such a feature should be prepared ahead of time.