It uses default GTR2 sounds found in the \GameData\Sounds\GTR\ folder.............
SPRINT3SOUNDS.aud file in the mod:
// inside coasting
VS_INSIDE_COAST_ENGINE_1=GTR\vette_idle.wav
VS_INSIDE_COAST_ENGINE_2=GTR\vette_offverylow.wav
VS_INSIDE_COAST_ENGINE_3=GTR\vette_offlow.wav
VS_INSIDE_COAST_ENGINE_4=GTR\vette_offmid.wav
VS_INSIDE_COAST_ENGINE_5=GTR\vette_offhigh.wav .....are you missing the .aud file?