Textures are all in Portable Network Graphic (PNG) format. Graphics can be 24 or 32 bit color, but if you don't need the Alpha channel leave it out. We prefer to produce these graphics using The GIMP, and many of the originals in the Art repository are in GIMP's native format (XCF). Other originals are created in Inkscape using its native Scalable Vector Graphics (SVG) format.
Models used in the game are all JOE format, which is produced by a Python script for Blender3D. Currently models are used for cars (glass, interiors, wheels, etc.) and track objects. Get the modeling tools from the tools directory of the Art repository. There are two JOE export scripts, export-joe-0.3.py and export-all-joe-0.3.py.
Sounds are in PCM Waveform (WAV) format which can be editing using any waveform audio editor such as Audacity.