http://www.nytimes.com/2008/10/02/techn ... skype.html
http://www.theregister.co.uk/2009/02/12 ... pe_pwnage/
I've never personally looked into such things with skype myself.
If the stream is in fact encrypted with 256 bit encryption, you can probably forget about hearing it, unless you allow it to reach the other end, get decrypted, and then try the audio driver approach. The problem there is that you'd ultimately get bunch of other sounds, music, games, movies as well, but it'd work , at least in theory.
MiTM attacks can be used to snoop and forge keys from a client to a host effectively becoming the streaming source and just forwarding it to the victim client. However, these attacks are relatively advanced and require a bit of expertise in terms of knowledge with the actual encryption scheme. Much like ettercap catches SSH1 keys to decrypt the stream (and trys to force client and server to use SSH1 via a downgrade attack), theoretically theirs not much you can do to prevent this. AloR (one of the devs for ettercap) claims that cracking SSH2 could be just as trivial only he hasn't had the time to implement it (it's been 4+ years AloR wtf m8). The same can be said with just about any encryption scheme that does an over the wire key exchange.