Some people have been wondering how they would go about adding the orchestral soundtrack to Dragon Quest VIII, so here's how I did it (you'll need both the NA (or EU) and Japanese versions of the game):
1. Dump your cart/carts to .CIA format using the Gamecart Dumper option in Hourglass9 (accessible by holding Start while powering on your 3DS, assuming you followed
https://3ds.guide to install CFW). If you have either version through the eShop, you'll have to figure out how to dump those yourself, since I'm not familiar with that process.
2. Copy the .CIA file(s) you made from your SD card (Hourglass9 places them in the files9 folder) to your PC. Use
HackingToolkit3DS to extract the files from both .CIAs.
3. The file structure is the same for both - the audio files are in the "ExtractedRomFS / rom / Sound / stream" folder. The soundtrack in the Japanese version consists of the files ranging from "BG_001.pcm16.bcstm" to "BG_046.pcm16.bcstm" (The BG files actually continue up to BG_077, but the rest are small jingles and sound effects, and replacing those leads to the end-of-battle and save jingles being way too loud for some reason). In the NA/EU version, the filenames are slightly different - "BG_001.dspadpcm.bcstm", for example. So rename the Japanese soundtrack to match the NA files, then replace the NA/EU soundtrack with the renamed JP files. Also, BG_041 was present in the JP version but not the NA one, at least for me. I left that one out of the transfer, since I'm not sure what would have happened if I added a file that wasn't originally there.
4. Using HackingToolkit3DS again, rebuild the NA/EU .CIA, then put it on your SD card, and use FBI (a homebrew app you should already have on your 3DS if you followed
https://3ds.guide) to install it.
5. If you've already started playing the game, you'll probably want to continue your save file. You can use
JK's Save Manager to back up your save and then import into your new modded version.
I'm no expert at this - I only recently installed CFW myself. It's possible I didn't do everything in the ideal way, but it worked for me!
Also, if you were interested in replacing the voice acting with the Japanese version, you should be able to do that as well, since all the audio files are in the stream folder. I didn't try, since I like the English VA.
If you need more help with any step in the process, you can ask here, or there are tutorials you can find online for each app/program I used - that's how I figured this stuff out myself, after all.