As of the time of writing (October of 2016), we don’t currently support changing playback speed it in the Mixer. The reason for this is that such an effect would need to recursively affect the speed of every event that routes into the affected bus, and the existence of sends, returns, nested events, and the transceiver effect make determining which event instances should be affected and by how much a complex problem. We are working a solution, but we can’t yet accurately predict when it will be finished.
For now, you can apply pitch-free playback speed adjustment to events by adjusting each event’s pitch in the macro controls and then adjusting it back with the FMOD Pitch Shifter effect. (You’ll want to control both kinds of pitch adjustment through automation on the same parameter.) Because the Pitch macro control changes the pitch with time stretching and the Pitch Shifter effect changes the pitch without time stretching, using one to cancel out the other should result in the playback speed being changed without any effect on the pitch.
Incidentally, rather than changing playback time of events when time is slowed, many sound designers use automation to alter the behaviour of their events without without any change to playback speed. For example, you could use an automated multiband EQ effect to apply an adjustable lowpass filter to your game’s music, making it sound muffled and distant. Of course, this works best when your game’s music is non-diagetic, or when your game’s setting permits diagetic music to be unaffected by in-game time shifts.