Objects can be added in the "Objects" mode which can be activated by the MAKEITGOOD cheat.
Most objects consist of model files (.m). They share the same structure with .prm files. These are for instance helicopters, boats, sprinklers, ....
As these objects perform several movements, many custom track makers replace them with other models. For instance, human added rotating fans and animated shadows to his track 'Jailhouse Rock' and Marv added flying birds to 'Fiddlers on the Roof '. Re-Volt 1.2 adds support for custom models. Those can then be put in a track-specific folder called 'custom' (ie /jailhouse/custom/copter.m).
However, there are also sound objects and emitters.