Is this even possible? I'm trying to add landing sounds for power armors only, but I have no idea if this can be done.

Even if you could do that, which I don't know, I doubt you could change one attached to the player like that. You can, on the other hand, use IsAnimPlaying JumpLand to check when the player is in the middle of landing from a jump, and then use PlaySound to play the sound file you want.

It may very well be possible to do it the way you suggest, though. I believe the most advanced werewolf mod available for Oblivion did something similar and replaced the animations used by the player only while transformed. If it can be done, it certainly wouldn't be anywhere near as easy.

Unfortunately Oblivion script extender, OBSE is much more advanced than NVSE. I'll try with IsAnimPlaying, thanks.

EDIT: Nah. IsAnimPlaying used in quest script with Begin GameMode block is too limited by FPS. Even if I manage to synchronize it, still this plays along with default landing sound, and it sounds terrible.
Looks like this feature cannot be implemented correctly in New Vegas.