Converting eac3 to aac and ac3

[dbzbleach]

I have a 2.67 gb mkv video. I'm trying to convert the audio to aac and/or ac3(stereo for both. I don't have any surround sound stuff so I'm assuming I have 2 channels) and I need a second opinion on the bitrate. I was thinking either aac or ac3 with a bitrate of 256kb/s. The original file is eac3 with a bitrate of 768 kb/s, 48.0 Hz, 6 channels. Also, does the video bitrate matter when choosing the audio bitrate? If so, the original video bitrate is 6943 kb/s. I was going to convert it to 19 constant quality with the same frame rate as the source(23.976). I'm keeping the container mkv. I'm using vidcoder and this conversion is so I will be able to watch it on my ps4 media player.

[davexnet]

Avidemux. Leave video as "copy" set the audio as required