add new sound volume cvars

[braem]

snd_volume_* cvars are all newly added. Make sure to reference the appropriate folder for each:

• snd_volume_menumusic is for sounds with mainmenu as a substring
• snd_volume_music is for sounds in momentum/sound/music/
• snd_volume_ambient is for sounds in momentum/sound/ambient/
• snd_volume_weapon_player is for sounds in momentum/sound/weapons/ that originate from the local player
• snd_volume_weapon_overall is for any sounds in momentum/sound/weapons/
• snd_volume_ui is for sounds in momentum/sound/ui/
• snd_volume_physics is for sounds in momentum/sound/physics/
• snd_volume_footsteps is for sounds in momentum/sound/footsteps/