The subject behavior is the result of a timing issue that causes 1 or 2 audio packets to NOT be rendered when you use RTSP, the Fast Cache feature, and client-side playlists.

Download the appropriate fix:

Windows Media Player 9 Series for Windows XP

Download the WindowsXP-KB884267-x86-ENU.exe package.

Windows Media Player 9 Series for Windows 2000 and Windows Server 2003

Download the WindowsMedia9-KB884267-x86-ENU.exe package.

Windows Media Player 10

Download the WindowsMedia10-KB884267-x86-ENU.exe package.