*solved* as noted in the posts below, the music is included in mp3 format
As the topic said. I noticed a low download size, so does that mean the game comes without in-game music or has it been converted for mp3 playback?

Good catch - I didn't correlate the download size with the cd-music. If no music comes with it - this might be a deal breaker for me.

I don't think it would be easy to change the game (and why when it works). But if GOG decides to lower the amount of data then it may offer flacs. So my guess is there will be the original CD music (it is not worth the work).
The game would not be complete without the music. And in my opinion GOG would not dare offer us incomplete game.
DON'T PANIC!

Maybe the music won't be included when the game is released BUT it wil come along as bonus content once GOG gets the permission to release/use the music. Remember, they put out additional content AT NO EXTRA COST to you all the time.

Although the download size may be small, look at the requirements. It shows 4GB as being the necessary size. Considering 1 through 3 are relatively small, I think this may bode well for 4 through 6. It's possible that the data is just heavily compressed, or is not finalized.

Maybe GOG uses a new bundling procedure like those we found some years ago from the zero-day scene with files compressed using specialized algorythms : uncompressed audio encoded to mp3, images in bitmap format converted to jpeg, etc.
Installers always took ages to run as all files were uncompressed and then repacked to the original game format :)

Maybe 450M is not the final number. I've checked the original disks. The game installation on the first disk is ~420M. Classic games are around 68M altogether. The rest of the first disk is filled by yet another copy of DirectX, four demos, and multiple installers. In case if anybody is curious, the demos are Army Men, High Heat, Meridian 59, and Requiem. the second disk contains 200M of Smacker videos and 15 audio tracks. I suspect that the second disk can be recompressed much better and everything but demos would neatly fit one CD. Speaking of CDs, the game came on those cool holographic CDs with 3D dragon head. One of those cooler packages.
Just for fun. Here are two covers from the original boxes. One is from the original US release (Limited Edition), another one is later Special Edition (could not resist Fry's $9.99 price about year ago, never actually opened the box). Those two boxes... it's like a puzzle -- find 10 differences. Just open both images and judge for yourself. Interesting enough, the Special Edition box looks closer to the game poster, just heavily cropped, to the point that there is no artist's signature. Totally bizarre.

I actually had trouble getting the music to play in MM6. But what I failed to realise, is you have to launch "gogwrap.exe" as administrator "I'm using Windows 7 x64" and then when it's loaded, launch MM6.exe then you will have music in MM6 :)

Music is in the game. Yay!

If you check in the game folder the audio is there in MP3 format, hence the smaller file size.

hmmm, yes music are here, but they are some bug. I 've check with my old game M&M VI French edition and the music for "Sorpigal" is not the good track number...