Windows Media Services does not support variable-bit-rate .mp3 files. If your .mp3 file was encoded using a variable bit rate, it will not play back as expected when it is streamed from Windows Media Services.
Another problem may be that your .mp3 file content is too short. In order to ensure reliable rendering on certain versions of Windows Media Player, content used with Windows Media Services must meet or exceed the minimum content length supported by the Player. The minimum supported content length Windows Media Player 9 Series or later is 5 seconds. The minimum supported content length for previous versions of Windows Media Player is 30 seconds. Content that is shorter than this minimum length may not render completely or at all.