Battlemap unit voices are controlled by two files, EDU (export_descr_unit) and EDSUV (export_descr_sounds_unit_voice). Using your Epirote bodyguard as an example, in EDU the key fields are "accent" (West_Greek), "voice_type" (General) and "type" (hellenistic cavalry molosson agema bodyguard). In EDSUV, each voice mod is set up as an "accent", and there are three "voice_types" for each accent (General, Heavy, & Light). Under the "General" section there is an entry for "hellenistic cavalry molosson agema bodyguard", and in fact it has two voice files, any one of which might play when you click on that unit. The generic Hellenistic Bodyguard "type" is "hellenistic cavalry general bodyguard", and is used by 4 factions. It too is listed in EDSUV, but only has one voice file. Also, because the "type" is used by makedonia, seleukid, ptolemaioi, and pergamon, you'll get the same voice file for all four factions.
At the end of the day, there can only be one voice (albeit perhaps multiple sound files that use different intonations) per unit "type" (as controlled by the EDU setting), and since there's a maximum of 500 units, that is also going to be the upper limit for unique individual unit voices on the battlefield.
Edit: Looking specifically at the Agema Phalangitai, there is one unit "type" (hellenistic infantry agema phalangitai) which is assigned to 5 factions, including the Seleukids and Epirotes. So in that case, different voices (as in your example) won't be possible.