The Microsoft DirectX End-User Runtime installs a number of runtime libraries from the legacy DirectX SDK for some games that use D3DX9, D3DX10, D3DX11, XAudio 2.7, XInput 1.3, XACT, and/or Managed DirectX 1.1. Note that this package does not modify the DirectX Runtime installed on your Windows OS in any way.
DirectX is a set of components in Windows that allows software, primarily and especially games, to work directly with your video and audio hardware. Games that use DirectX can use multimedia accelerator features built-in to your hardware more efficiently which improves your overall multimedia experience.
While DirectX governs both video and audio, these are the features of DirectX that you can change.