They are already available as virtual key values:
Volume Mute key = 173
Volume Down key = 174
Volume Up key = 175
Next Track key = 176
Previous Track key = 177
Stop Media key = 178
Play/Pause Media key = 179
As noted on VoiceAttackHelp.pdf page 158, you can use a virtual key value enclosed in brackets for variable hotkeys, E.G.
Set Text [~key] to '[179]'
Press variable key(s) [~key] and hold for 0,06 seconds and release
will press media play/pause.