DirectX Audio is a blanket term that refers to DirectSound and DirectSound3D, which are included in the DirectX bundle of application programming interfaces (APIs).
Other DirectX related glossary:
DirectDraw
Direct2D
Direct3D
DXGI
DirectInput
DirectPlay
DirectPlay Voice
DirectSound
DirectSound3D
DirectMusic
DirectX Audio
DirectX Media
DxDiag
DirectX Media Objects
DirectSetup
XAudio2
