All of the HUD drawing code is in the VAMOSWORLD:

rawHUD function in vamosworld.cpp. The VAMOSWORLD class is unfortunately a messy, miscellaneous class full of random junk, but the DrawHUD function is relatively straight forward. You can draw a texture as a 2D object with the game.utility.Draw2D function, passing in normalized x & y screen coordinates (so, ranging from 0 to 1) for the upper left and lower right corners, and then a TEXTURE_HANDLE object.
The sound classes are in sound.cpp and are pretty simple to use. You ask the SOUND object (game.sound) for a new sound source (SOUND::NewSource) giving a path to the .wav or .ogg sound file (which is just a buffername), and then you use the SOUNDSOURCE class's public interfaces to control the properties of the sound source (like, to play and stop it, to reposition it in the world, or to turn off the 3D positioning).
What sort of HUD/audio projects did you have in mind?