I also had this problem and fixed it according to this thread and setting this value to 85. FYI, when I downloaded the game the default setting for this value was 850, not 85! No wonder the game went crazy...
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It was a regional problem. The value in the options file is 0,85 with a comma as the decimal sign. On your system the decimal sign is probably a point, so Unity skips the decimal sign, multiplying the value in the process. You probably have a similar setting for the traffic car engine sounds, which makes them way too loud.
It's fixed in the latest version.