Can I copyEntity a 3dsound?

No. Sounds aren't entities.

I'm not sure why you would want to copy a sound - you can't modify it so why not just use the original?

You can do this though:

sound = Loadsound("MySound.wav")
blah = sound
PlaySound blah

Then add these two lines - you'll see that it generates an error because 'blah' and 'sound' both point to the same sound:

Freesound blah
Playsound sound

Perhaps this is what you're trying to do?