Sony have finally (after fans have pleaded since launch) revealed as to why they havent implemented the much requested cross-game chat. According to Sony Worldwide Studios president Shuhei Yoshida its an issue with games being given priority when distributing the RAM. This doesnt leave anything over to perform any other tasks such as voice chat. It also cannot be changed or implemented in any way.
Once a game gets RAM we never give it back. It's not possible to retrofit something like that after the fact. The game has to use its own memory to do in-game voice chat. There's always voice chat in the game. But it's a part of a game feature. It's not a part of an OS feature. That's the reason in terms of the ability to have voice chat across different games.