The MetroSafe system that you are listening to for LMPD, is a simulcast system. Because you are probably receiving a couple of sites at the same time, your scanner is having trouble with the decoding the multiple signals and you get some garble. (There's lots of posts on this subject elsewhere).
At the risk of confusing a new scanner user, let me offer up a suggestion to try.
You can adjust the P25 Adjust Mode and P25 Adjust Level which may improve things. To do so, turn the scanner off. Then while holding down the "HOLD/RESUME" button, power the scanner back up. Go to SETTINGS>P25 ADJUST MODE. Change the number to 13 and hit enter. Then go to P25 ADJUST LEVEL and change that number to 175. (Probably wouldn't hurt to note the original settings just in case these new values don't make things better and you need to return to the default.)
I listen to the MetroSafe system regularly. Out in my part of the county, the 13 and 175 values seem to give me the least amount of "garble", though there is still some present.
You may also wish to check which firmware version your scanner has. If it isn't the 1.07.03 version listed toward the top of the Uniden forum, you may wish to download the 1.07.03. (I found the 1.07.03 version made some improvement on the MetroSafe system, but not nearly as much as the 396XT firmware update did on the same system).
An external speaker will not improve the audio. I've got my fingers crossed that Uniden will come up with another firmware update that will make the 996XT sound as good on simulcast systems as the firmware they did for the 396XT.
It's also worth noting for the benefit of the new scanner user that a number of police talkgroups (LMPD and others) are encrypted and your scanner won't receive them. These are noted by an "E" in the Mode section of the RR database. There are a couple of different encryption systems in use (I won't confuse you with more on this). Depending on which is used, your scanner may stop on a talkgroup and you'll hear some weird sounds - nothing intelligible.