Imagine this: You are walking through a swamp, and all you can hear is the soundtrack. Now imagine this: You are walking through a swamp, and you can hear crickets chirping. Which one is better? That is why I suggest custom biome ambience. I haven't expanded on this much because I mainly have a writers block, but I would expand on this over time.
So... this is something that is only possible through resource packs. Community members are more than welcome to work on and share resource packs; but there are no current plans for EMC to head such an effort at this time.
There are some ambience sounds that occur in Minecraft for things like caves (see wiki page) but kitten is saying that he'd like to add custom biome sounds to biomes rather than just have the usual sounds of mobs, some music, and the player that you get in all biomes. And yeah, as they've said before, it would be better to just achieve this with a personal resource pack rather than attempting to add it to the whole server.
Yes, but how would you make specific sounds play in specific biomes instead of randomly when there's a cave nearby?