Extract from the Audio manual entry :
The game timescale can be used to speed up or slow down playback of the game, for effects like slow-motion. See Delta-time and framerate independence for more information. This property controls whether or not audio is affected by the game's timescale.
Off will play back audio the same regardless of the timescale.
On (sounds only) will play back audio from the Sounds project folder at a different rate depending on the timescale, but will always play back audio from the Music project folder at the same rate.
On (sounds and music) will play back all audio at a different rate depending on the timescale.
Some browsers may not support audio timescaling at all; test on multiple browsers to establish support. Also, different browsers may use different time-stretching algortihms for audio. Browsers can either alter pitch, going to lower pitch for lower time scales like a tape machine playing at a different speed; alternatively they can change the playback speed without affecting the pitch. Test on multiple browsers to establish the exact effect achieved.
Are you sure you've setted the property to "on" ?
What is the browser you're testing this in ?