I strongly dislike the new art in the SE's, and I hate that I can't start a game and watch the intro and hear the music using the original graphics and sound. When it automatically switched to the new graphics in the intro sequence, it's a very deflating moment, for me.

So, I'm wondering if the GoG versions include the original DOS releases as bonuses.

They don't.

However, if you're on Windows or have access to a Windows PC, you could check out the Monkey Island Ultimate Talkie Edition Builders from LogicDeLuxe. These builders require the Special Editions, and will extract the classic editions that are packed into them, add music, speech and sound effects (from the Special Edition), so that you can play them fully via ScummVM (including original intro and credits).

The classic versions that the SE pack in their resources are the VGA CD ones for the games. With the Ultimate Talkies, you have the option to choose which music you want the games to play, but, as of yet anyway, the sound effects are from the SE editions.

With this method, is there an option to not add the new voiceovers, but still have the original intro and credits?

You can silence or disable speech while playing so you'll get the classic version without voices, but with the other options available (for music and ambient sounds).

Thanks, I look forward to trying that out. I have yet to play through the series so I'd like it to be as close to the original experience as possible.

For the original feeling you should also try and get the EGA version of Monkey Island 1, which was the original one. It also had a different interface than the VGA version, the original interface looks like the one of Indy3.
The dithering technique was introduced in Loom, in the original EGA version of MI1 they went crazy with it.
https://i.ytimg.com/vi/LY_WtfjQ1FE/hqdefault.jpg
https://videospielhalbwissen.de/wp-content/uploads/2018/11/timeline-monkeyisland.jpg
https://scientificgamer.com/blog/wp-content/uploads/2020/11/mi_herman.png

I'd say in some cases the EGA version looks even better than the VGA version (in VGA the sunset is gone).

Another version worth having is the Amiga version. It uses a 32 color version of the VGA backgrounds and the sprites from the EGA version. The interface is the original one (same as the EGA version)
The music uses the MOD format which is only second to a good general midi soundcard and the intro music was created by Chris Hülsbeck, who altered a sequence in the title theme a little bit.

Not sure if this is what you want, but you can always switch to the classic version within the game. This works from the very beginning of the game, including the intro. You can switch by pressing [F1] for Monkey Island 2, I think it is [F10] for Monkey Island 1.

The correct key can also be found in in-game help.

So, no. The original DOS-version is not includes, but you can switch to original version (with original graphics and sounds) within the windows-version.

I am quite sure he was aware of that when he wrote "When it automatically switched [...]".
WIth the method mentioned above it is possible to get working DOS VGA versions, not the original EGA version however.

Maybe a little late, but I also was very dissappointed with this release.

I wanted the German version with the original subtitles, not the newly translated ones though they might be better, I don't know. But I wanted to create the original feeling, and I have a soft spot for the old translation. It's what I grew up with.

So the SE is, for me at least, worthless.

Now here's my tip: I got the original CD release of Monkey Island 1 on ebay for about 5 Euros, without packaging. It was only the jewel case, no big box, but that was all I needed. If you are not looking for a fully boxed version you can get that pretty cheap.

I made an ISO of it, with everything (data + Audio tracks). In DosBOX it's possible to mount that ISO as a CD-Drive. And behold, original graphics and the fantastic soundtrack from the CD, without problems. Might work in ScummVM too, I haven't tried that.

So maybe you want to go that route.