Audio in RescueHQ is implemented using FMOD. Use FMOD Studio (version 1.10.0, Free Indie License) to create audio assets (FMOD banks) for the game (see http://www.fmod.com).
It is recommended to use the FMOD Template Project provided in the official mod assets repository and go from there. This project contains the mixer setup used by the game, which allows you to route your sound events to the correct destinations. When opening the project for the first time, FMOD Studio will show a load warning, allowing you to "recover" the project. This is expected and will simply generate a new master bank id for your sounds.
FMOD allows you to define events and give them names.
The game looks up these events by name. If there is more than one event with the same name, the event from the mod loaded last is used. This allows mods to override sounds of mods loaded before.
Sounds created using FMOD Studio must be built in order for the game to use them. Set up your FMOD Studio project so that the output folder for built files is anywhere inside your mod folder (your source FMOD Studio project can be anywhere you like, it does not need to be inside your mod folder) The game will automatically load any FMOD audio banks (*.bank files) it can find inside your mod (except those for voiceovers)
Built voiceover banks are loaded by the game only if the language matches, and must be named "VO_<language>.bank".
For example, if the voiceover language in the game is set to English (EN), the voiceover bank that the game attempts to load must be named "VO_EN.bank"
Voiceovers work using audio tables and a programmer instrument (please refer to the FMOD documentation for details).
A voiceover sound bank just uses that audio table, and needs a sound event named "VO/<language>" (eg. "VO/EN") containing a programmer instrument element.
The key names inside the audio table should usually match the text localization key that the audio represents.