SRS is no longer available in FMOD, we replaced it with a Dolby PL2 downmix, however that downmix is opt-in via FMOD_INIT_PREFER_DOLBY_DOWNMIX. The default downmix is a simple mix matrix performed by either the Core API or the Studio API.
When the master bus is set to 5.1 and the output device is stereo, the Core API performs a downmix like this:
{ 1, 0, 0.707, 0, 0.707, 0 }
{ 0, 1, 0.707, 0, 0, 0.707 }
When a Studio bus takes 5.1 input and is forced to stereo output the Studio API performs a downmix like the following. This is equivalent to adding a panner DSP effect with 5.1 input and stereo output (center panned).
{ 0.720, 0, 0.360, 0.499, 0.578, 0.129 }
{ 0, 0.720, 0.360, 0.499, 0.129, 0.578 }