These are all possible strings that can be passed to PlaySound().
Keep in mind that many of them are obsolete and may either not have any sound attached and/or may go away at any time.

GLUESCREENSMALLBUTTONMOUSEDOWN
GLUESCREENSMALLBUTTONMOUSEUP
GLUESCREENSMALLBUTTONMOUSEOVER
GLUESCREENMEDIUMBUTTONMOUSEDOWN
GLUESCREENMEDIUMBUTTONMOUSEUP
GLUESCREENMEDIUMBUTTONMOUSEOVER
GLUESCREENLARGEBUTTONMOUSEDOWN
GLUESCREENLARGEBUTTONMOUSEUP
GLUESCREENLARGEBUTTONMOUSEOVER
GLUESCREENEDITBOXKEYCLICK
GLUECHECKBOXMOUSEDOWN
GLUECHECKBOXMOUSEUP
GLUECHECKBOXMOUSEOVER
GLUECHARCUSTOMIZATIONMOUSEDOWN
GLUECHARCUSTOMIZATIONMOUSEUP
GLUECHARCUSTOMIZATIONMOUSEOVER
GLUESCROLLBUTTONMOUSEDOWN
GLUESCROLLBUTTONMOUSEUP
GLUESCROLLBUTTONMOUSEOVER
GAMEABILITYBUTTONMOUSEDOWN
GAMESPELLBUTTONMOUSEDOWN
GAMEWINDOWOPEN
GAMEWINDOWCLOSE
GAMEDIALOGOPEN
GAMEDIALOGCLOSE
GAMENEWWINDOWTAB
GAMESCREENSMALLBUTTONMOUSEDOWN
GAMESCREENSMALLBUTTONMOUSEUP
GAMESCREENSMALLBUTTONMOUSEOVER
GAMESCREENMEDIUMBUTTONMOUSEDOWN
GAMESCREENMEDIUMBUTTONMOUSEUP
GAMESCREENMEDIUMBUTTONMOUSEOVER
GAMESCREENLARGEBUTTONMOUSEDOWN
GAMESCREENLARGEBUTTONMOUSEUP
GAMESCREENLARGEBUTTONMOUSEOVER
GAMETARGETFRIENDLYUNIT
GAMETARGETHOSTILEUNIT
GAMETARGETNEUTRALUNIT
GAMEHIGHLIGHTFRIENDLYUNIT
GAMEHIGHLIGHTHOSTILEUNIT
GAMEHIGHLIGHTNEUTRALUNIT
GAMEINITIALATTACK
GAMEERROROUTOFRANGE
GAMEERROROUTOFMANA
GAMEERRORUNABLETOEQUIP
GAMEERRORINVALIDTARGET
ACTIONBARBUTTONDOWN
MAINBUTTONBARMENU
MINIMAPZOOMOUT
MINIMAPZOOMIN
MINIMAPOPEN
MINIMAPCLOSE
BAGMENUBUTTONPRESS
LOOTWINDOWOPEN
LOOTWINDOWCLOSE
LOOTWINDOWCOINSOUND
ITEMWEAPONSOUND
ITEMARMORSOUND
ITEMGENERICSOUND
LEVELUPSOUND
GLUECREATECHARACTERBUTTON
GLUEENTERWORLDBUTTON
SPELLBOOKOPEN
SPELLBOOKCLOSE
SPELLBOOKCHANGEPAGE
PAPERDOLLOPEN
PAPERDOLLCLOSE
QUESTADDED
QUESTCOMPLETED
QUESTLOGOPEN
QUESTLOGCLOSE
GLUEGENERICBUTTONPRESS
GAMEGENERICBUTTONPRESS
INTERFACESOUND_MONEYFRAMEOPEN
INTERFACESOUND_MONEYFRAMECLOSE
INTERFACESOUND_CHARWINDOWOPEN
INTERFACESOUND_CHARWINDOWCLOSE
INTERFACESOUND_CHARWINDOWTAB
INTERFACESOUND_GAMEMENUOPEN
INTERFACESOUND_GAMEMENUCLOSE
INTERFACESOUND_LOSTTARGETUNIT
INTERFACESOUND_BACKPACKOPEN
INTERFACESOUND_BACKPACKCLOSE
INTERFACESOUND_GAMESCROLLBUTTON
INTERFACESOUND_CURSORGRABOBJECT
INTERFACESOUND_CURSORDROPOBJECT
SHEATHINGSHIELDSHEATHE
SHEATHINGWOODWEAPONSHEATHE
SHEATHINGMETALWEAPONSHEATHE
SHEATHINGWOODWEAPONUNSHEATHE
SHEATHINGMETALWEAPONUNSHEATHE
SHEATHINGSHIELDUNSHEATHE
igCreatureAggroDeselect
igQuestListOpen
igQuestListClose
igQuestListSelect
igQuestListComplete
igQuestCancel
igPlayerInvite
igPlayerInviteAccept
igPlayerInviteDecline
GAMEERRORUNABLETOEQUIP
ITEMGENERICSOUND
GAMEERRORINVALIDTARGET
LEVELUP
GAMEERROROUTOFRANGE
QUESTADDED
MONEYFRAMEOPEN
MONEYFRAMECLOSE
LOOTWINDOWOPEN
LOOTWINDOWCLOSE
LOOTWINDOWCOINSOUND
GAMEHIGHLIGHTHOSTILEUNIT
GAMEHIGHLIGHTNEUTRALUNIT
GAMEHIGHLIGHTFRIENDLYUNIT
INTERFACESOUND_LOSTTARGETUNIT
INTERFACESOUND_CURSORGRABOBJECT
INTERFACESOUND_CURSORDROPOBJECT
GAMESCREENMEDIUMBUTTONMOUSEDOWN
GAMEABILITYACTIVATE
GAMESPELLACTIVATE
gsTitleEnterWorld
gsTitleOptions
gsTitleQuit
gsTitleCredits
gsTitleIntroMovie
gsTitleOptionScreenResolution
gsTitleOption16bit
gsTitleOption32bit
gsTitleOptionOpenGL
gsTitleOptionDirect3D
gsTitleOptionFullScreenMode
gsTitleOptionOK
gsTitleOptionExit
gsLogin
gsLoginNewAccount
gsLoginChangeRealm
gsLoginExit
gsLoginChangeRealmOK
gsLoginChangeRealmSelect
gsLoginChangeRealmCancel
gsCharacterSelection
gsCharacterSelectionEnterWorld
gsCharacterSelectionDelCharacter
gsCharacterSelectionAcctOptions
gsCharacterSelectionExit
gsCharacterSelectionCreateNew
gsCharacterCreationClass
gsCharacterCreationRace
gsCharacterCreationGender
gsCharacterCreationLook
gsCharacterCreationCreateChar
gsCharacterCreationCancel
igCurrentActiveSpell
igMiniMapOpen
igMiniMapClose
igMiniMapZoomIn
igMiniMapZoomOut
igChatEmoteButton
igChatScrollUp
igChatScrollDown
igChatBottom
igSpellBookOpen
igSpellBookClose
igSpellBokPageTurn
igSpellBookSpellIconPickup
igSpellBookSpellIconDrop
igAbilityOpen
igAbilityClose
igAbiliityPageTurn
igAbilityIconPickup
igAbilityIconDrop
igCharacterInfoOpen
igCharacterInfoClose
igCharacterInfoTab
igCharacterInfoScrollUp
igCharacterInfoScrollDown
igQuestLogOpen
igQuestLogClose
igQuestLogAbandonQuest
igQuestFailed
igSocialOepn
igSocialClose
igMainMenuOpen
igMainMenuClose
igMainMenuOption
igMainMenuLogout
igMainMenuQuit
igMainMenuContinue
igMainMenuOptionCheckBoxOn
igMainMenuOptionCheckBoxOff
igMainMenuOptionFaerTab
igInventoryOepn
igInventoryClose
igInventoryRotateCharacter
igBackPackOpen
igBackPackClose
igBackPackCoinSelect
igBackPackCoinOK
igBackPackCoinCancel
igCharacterNPCSelect
igCharacterNPCDeselect
igCharacterSelect
igCharacterDeselect
igCreatureNeutralSelect
igCreatureNeutralDeselect
igCreatureAggroSelect
UChatScrollButton
Deathbind Sound
LOOTWINDOWOPENEMPTY
TaxiNodeDiscovered
UnwrapGift
TellMessage
WriteQuest
MapPing
igBonusBarOpen
FriendJoinGame
Fishing Reel in
HumanExploration
OrcExploration
UndeadExploration
TaurenExploration
TrollExploration
NightElfExploration
GnomeExploration
DwarfExploration
igPVPUpdate