The alleged radio contact with MAS370 made by the anonymous captain of a Japan-bound airliner makes me smell rats. Why should a real pilot with a verifiable record refuse to give his own name and his flight number in such a situation? What's the problem with it? Wouldn't this help the investigation? His alleged statement is also highly suspicious. He heard nothing, all he says is that "there were a lot of interference… static… but I heard mumbling". In short, he is unable to refer the content of the transmission, he is unable to say whether he spoke with the captain or the F/O, the alleged time of the radio transmission is after the time the datalink had been turned off and the transponder had been turned off. Sorry, but to me this smells like a typical piece of disinformation. Someone planted this interview just to "prove" that the captain and the F/O were still at the controls of the aircraft at that time. I will believe this captain as soon as he will come out with a real name and the exact position of his aircraft, which should not be so difficult to verify with a map and radar data.
I really wouldn't read too much into that one. I was on frequency at the time, I heard the other MH aircraft transmitting on 121.5 trying to contact the MH370 (along with many transmissions from HCM control) and never heard anything resembling a reply, mumbled garbled or otherwise.
There's a fairly common interference phenomena around SGN that seems to cause short 5-10 second bursts of buzzing static on VHF. He might have heard that, there was plenty of that going on that night but nothing out of the ordinary.