What components, wiring, and electronics needed for helmet internals
One option you have is to use off-the-shelf items rather than to try to build your own from components. So many different options are available for the objective you describe, but I think I've found one that fits well with your use.

It's a wired intercom kit that uses a 9V battery and a pair of wires to connect one station to the other. The $19.95 price is for one unit, you'd have to buy two for your purposes.



The microphone is the silver cylinder in the lower left corner of the circuit board and could be extended to the outside (ear?) of the helmet, while the board and speaker would be mounted inside. If you wanted to go slightly overboard, get one set for each ear and one for you, with the speaker at the outside mouth location and the board and microphone inside near your mouth.

As a kit, you'd have to be able to solder the components to the board, but that's a good start in the maker world and a skill that may serve you again in the future. The kit is a good one for learning to solder, as there are few components and nothing too small or complex.

Unfortunately, the instruction manual does not include part identification, nor sequence for assembly, and is therefore lacking as a true tutorial for soldering, but it is not advertised as such. It should still be possible for a novice if appropriate searches are made online for the polarized components.