Shrink AC3 stream size?

[zmin]

hi, i'm trying to strip down an mkv by about 100mB to fit it on a dvd and i'm thinking the easiest way to do that is to convert the ac3 stream
the format is 5.1 at 384kbps, which means about 64kbps per channel. the stream is 2 hours long and takes up about 336mB.

i'm thinking about two options
1) re-encode the whole ac3 to a bitrate of 56kbs (or just the rear 3 channels to 48kbps)
or
2) downmix the 5.1 channels to stereo or 2.1 (which i figure implies the ac3 matrix and that kinda complicates things for me)

my question is which one of the two is best / easier.
and what software to use and how to use it.

thanks!

Format : Matroska
File size : 4.48 GiB
Duration : 2h 2mn
Overall bit rate : 5 234 Kbps

Video
Format : AVC
Format/Info : Advanced Video Codec
Codec ID : V_MPEG4/ISO/AVC
Duration : 2h 2mn
Bit rate : 4 629 Kbps
Nominal bit rate : 4 846 Kbps
Width : 1 280 pixels
Height : 544 pixels
Frame rate : 25.000 fps
Resolution : 24 bits
Writing library : x264 core 65 r1046M 71d34b4

Audio
Format : AC-3
Format/Info : Audio Coding 3
Codec ID : A_AC3
Duration : 2h 2mn
Bit rate mode : Constant
Bit rate : 384 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Surround: L R, LFE
Sampling rate : 48.0 KHz

[Baldrick]

You could use avidemux or xvid4psp, open mkv, under video choose copy, under audio choose ac3, settings and change the bitrate, save as a new mkv. I would probably make it stereo if you want to use that low bitrate.