Just found your query again in Dan's forum, where I don't have an account...so here my guess:
Looks for me like a faulty audio-filter-setup of your Windows(XP?).
Some of this many (sound/video)codec-packs arround have some GUI, where a user can very easily screw-up his working sound setup.
In Dan's forum you mentioned, that this issue happens when you switch to external view (F1).
Maybe, you have set orbiter-sound to play music while in ext. view.
So, orbiter-sound "instructs" your OS, to play a music-file using OS-system-default-sound-settings for file-decoding.
Whatever happened to your system, it seems to be that your OS-default-decoder tries to send the decoded music-stream "through" the missing DLL.
The DLL cannot be found, so the OS (not Orbiter or Orbiter-Sound) complains about the missing/corrupted DLL.
Solution ? No clue :idk: Maybe:
If you have installed one of the many available sound/video-codec-packs...just try to open its GUI (if the pack has a GUI) and hit any "DEFAULT"-button you can find.
Important to know is, that even the possible fact, that an external whatever-mp3-player might play (without errors) your MP3's, this does not mean, you system-wide sound-config is in a good state.
Many MP3-player are using their own(good or bad) plugin-driven decoders.
Orbiter-Sound relies on your system-default-sound-setup.
Another option, just install the missing DLL and hope the best.
But I would prefer o fix your codecs-setup first.
Just a guess...I am not a Windows expert....