As soon as the SDK is out and mods enabled you can use as sound what you want. Even to make your Merlin sound like a Lawn-mower if you like. I don't see the need of the dev team to implement what somebody modded and which likely does not fit to the sound engine as implemented in the program.

I for my part rather like the sounds from the cockpit and they are not that bad except that there's no wind sounds. In reality a Merlin when one sits in the cockpit did not sound like what you hear on videos.

The outside sounds are definitely placeholders and will be replaced by sounds that will fit to the sound engine for default. You will be able to change this as soon as mods are allowed.