Hi,
After some research and test in steam forum with Valve employee post (Jay Stelly in particular). It appears that Alchemy is not usefull for all Valve game.
All non source games like half-life 1, counterstrike 1.6, hl1 mod etc.. :
alchemy is need to have full surround sound support and eax effect.
Half-life2
Half-life Source
Half-life Source deathmatch
Half-life Lost coast
Dark messiah
Sin emergence
Vampire Bloodlines
need alchemy to have Full surround sound.
Half-life2 episode one
counter-strike Source
Half-life2 deathmatch
Source SDK Base (use by mod like PVK 2.2, Eternal silence 3.1 and more)
Alchemy is optionnal (or useless ?), as you can restore full surround sound by typing in the console : snd_digital_surround 1
Just add +snd_digital_surround 1 in startup options of each games and mods (which use the source sdk base).
Half-life2 episode two
Team Fortress 2
Portal
Source sdk base -orange box (use by some source mod avalaible in steam (insurgency, synergy etc...)
left 4 dead
Day of defeat Source
Alchemy is optionnal (or useless ?) Full surround sound support without doing anything.
More Informations about :
snd_digital_surround 1 : This setting bypasses direct sound 3d for surround for a 6-channel digital stream which is more compatible with newer audio hardware.
The latest version of the engine (The Orange Box) and left4dead has this setting as the default.
snd_legacy_surround : (0 by def) : this setting only appear in orange box and recent source game like l4d, if set to 1, the game will use the older direct sound 3d path ( so snd_legacy_surround 1 = snd_digital_surround 0), so leave it at 0 and never change it to 1 in vista.
other information :
Half-life2, Half-life Source, Half-life Source, deathmatch, Half-life Lost coast, Dark messiah, Sin emergence, Vampire Bloodlines :
they all have the snd_digital_surround command but It can not be use to have surround sound, as the source audio engine for this game never been updated for Vista, in fact you can never set this command to 1 (alway return to 0), so alchemy is required.
that's all for now, I hope that will help you.
Choum