I discovered that some of the sound effects in the mod are in Stereo format, rather than Mono.
Ones I have discovered include:

• All the Lily Pad bounce sound effects (splash1.ogg, splash2.ogg, etc)
• Duck footsteps (step1.ogg)

The reason this is an issue is due to how the game treats the sound formats differently. Stereo sounds are reserved for ambient sounds and music tracks, meaning they play uniformly regardless of where the player is facing. While Mono sounds are directional, as these sounds are meant to be.