They are shown clearly in the later concept artwork in the Gallery section ... but in those images another symmetrical arrangement/set of holes is on the opposite side .... it seems they either filled in one set or preferred the 'asymmetric' effect of having only one set ... nothing to do with speakers i suspect.
At least one complete suit was built and assembled (late May, early June 1978) which served as a presentation prototype for approval:
The concept art you are talking about was drawn after or in time of the Supertrooper footage (28th June, 1978) when the Supertrooper idea was already dropped and replaced with the single bounty hunter, Boba Fett:
This drawing was more a generic concept and didn't reflected the real suit. Just to name some of the differences:
1. Switched L/R gauntlets.
2. Upside down knees with dart blocks on each sides.
3. Ammo pouches with different sizes and forms.
4. Symmetrical cheek (small holes between the upper and lower cheeks) and abdominal plate (the 3 holes) details.
5. Differently shaped armor plates.
There was another detail that appeared in the concept drawings, but changed its position during the prototype build:
This feature was not used in the pre-production, promo or hero suits.
So, what you see on those concept drawings is not a proof that it was used or intended to use on the real suit(s). But you might be also right regarding the 3 holes being present on both sides and later the right side filled in to achieve a non-symmetrical look.