[venomlivno8]

pawn Code:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_STATE_ONFOOT)
    {
        if(oldstate == PLAYER_STATE_DRIVER || oldstate == PLAYER_STATE_PASSENGER)
        {
            StopAudioStreamForPlayer(playerid);
        }
    }
    return 1;
}

Just add this to you're script.