Voicemail wav's sound fine until 6th sequential file is opened

I am totally new to the Asterisk universe and I am experimenting with a lab environment to get my feet wet. I have had great progress over the last week and this has been my first stumbling block
I am using two Mitel 5330 phones in SIP mode and a low cost Linksys switch to tie it all to a modestly powered Intel i7@3ghz machine running Ubuntu server.
I know the switching hardware is trash but I am only using two phones in this lab setup. I didn’t think two phones would really stress a testing environment.
I have voicemail conf set to utilize uLaw & aLaw.

Here is what I see on the console:

  1. I call into voicemail and everything sounds great
  2. I watch each wav segment open and play via the console
  3. When the sixth wav segment opens and begins to play it sounds like a robot.
  4. Any wav played from that point forward sounds like a robot regardless of length.

It always starts to degrade at the same point.
I am not real savvy with the entire range of tools that Asterisk may have to help troubleshoot this and I am confused that the initial wavs play so well.
I started to think maybe the Linksys switching hardware for the lab was the culprit but I added a softphone, via MicroSIP, on a distant machine and oddly enough the softphone does not have this issue. The softphone app is receiving streams on a slow laptop, connected via wifi, across the LAN, which first passes thru the crappy Linksys switch.
It makes me wonder if there is even a problem with the server.

On the 5330 phones I am running Mitel firmware ver 06.03.03.07 on Main and 06.04.01.02 on Boot.
On each phone I have the codec set to “G711uLaw & G711aLaw & G729A”.
I was assuming it would work from left to right in terms of preference.
I did run “show sip channels” on the console as it was streaming the VM prompts. It shows it was using uLaw under ‘FORMAT’.
Poking around the net, I read that Asterisk defaults to 30ms RTP Frame Size. The Mitel phone was set at 20. I changed it to 30 but the phones operation remain unchanged. It is really strange that it does not go sideways in the middle of a prompt. It sounds good when the 5th wav file opens then on the 6th it goes wacky.

Is there something else I should be looking at with the Mitel phones and their settings?

It seems as though the only way I was able to get the Mitel 5330 handsets to work is by setting the media page to use g729 codecs. I also installed the open source g729 codec in Asterisk version 16. Now the voicemail sounds normal. Seems to be a Mitel issue when using aLaw or uLaw. It may be an issue for the 5330 only. I do not have other types of handsets to test against though.