Set the type to skeleton and it'll still work correctly. It still plays the regular voice though, so you'll need to set the "Gagged" option and manually trigger lines with a logic_choreographed_scene / ambient_generic. Turrets have their OnRetire/OnActive outputs which you could use.

Ok... Not quite sure how the choreograph works. When I select a VCD it says no sounds in it. Also, it looks like there is one VCD for each thing the turret says. With that in mind, it looks like I'll need a big stack of choreographs or ambients for each turret. I've already found over a dozen things the turret could say at random OnDeploy.

Any chance I could maybe write some kind of script that will play a random voice line for the specified action? Or RunScriptCode from a logic_case?

There's a bug with Hammer that stops you hearing any of the voice lines, but they still work ingame. The Portal Wiki has a list of all the Defective Turret[theportalwiki.com] voice lines, which you can match with the VCDs. (The linked URL has the same filename as ingame).

Make a logic_case with a bunch of different outputs, then trigger the PickRandom input to play a random one. (If you use PickRandomShuffle, it'll only repeat a line after exhusting all the current lines.) If you have a lot of turrets, you might want to set the ambient_generics to Play Everywhere and then reuse them for every turret. Give the logic_cases names starting with '@', and you could put the customised turret in an instance to duplicate it easily. For the turret you might need to set the 'Load defective models' option if you get a 'model not precached' error on map startup.

The problem I see when re-using one sound emitter for all the turrets is that if more than one turret sees you only one will actually speak. True, if I have dozens of lines to play on dozens of "global" sound emitters, the chance of the same sound being selected would be sort of small. Although it would be fun to hear a chorus of "Identify yourself or I will shoot." ;)

Alternately you could just put them all in one instance with a turret and reuse that. To do the 'knocked over' lines, put a small trigger around the top of the turret. OnEndTouch the turret has fallen over and you can play some lines. If the turret is picked up Kill the trigger before it activates those lines (since they wouldn't match).

Well, I've found a nutty solution... And while putting these together I'm wondering if I shouldn't maybe turn them into mechanical worms with bazookas. XD

Ok, this was fun. When they explode, their debris are the white plastic covers of the ordinary turrets. I guess it would be too much of a hassle to exchange the models and/or scripts that handle what debris is to be used.

It's probably more that Valve didn't bother making special gib models for them, especially since they only ever actually explode once.