Alright, my second try at this guide. (Hopefully it doesn't get locked or I get banned in the process..) Hope you enjoy.

What you'll need:

DSLazy (I recommend DSBuff)
Copy of MMSF2 (English) of whatever version your going to use; Find it yourself!
Copy of RNR2 (Ryuusei no Rockman 2) to the corresponding game your using. (Zerker x Ninja get Berserk x Shinobi, Zerker x Sauryan get Berserk Dinosaur); Again, find it yourself!
Basic computer knowledge

1) Once you have the files, open DSBuff and open the japanese version of your game. Unpack everything and save the folder where it is easily accessible. Do not close DSBuff yet!

2) Open the US version of the game, and again save it where its easily accessible.

3) Go to your Japanese game folder, and find a folder that says "JP" or "COM". It holds the battle card data. Take the sound.sdat file out of there and put it where you can get it.

4) Go to the US game folder, and go to the same place where the sound.sdat file is.

5) Drag the Japanese sound.sdat from step 3 and drag it to the folder where the US sdat folder is. MAKE SURE IT OVERWRITES. IF IT DOESN'T YOU DID IT WRONG!!

6) Go back to DSBuff, and repack the US file. Save it whatever you want and put it on your flash cart.

7) If you did it correctly, you should hear one of the characters call out at the title screen "Ryuusei no Rockman 2: Berserk *
(Note: * symbol pertains to what version your playing)

FAQs:

Q: Does this hack mess up the game in any way?
A: As far as I know, no. I haven't had any reports of glitches.

Q: Whats so important about the voices?
A: There isn't anything important. US version doesn't have voices so I wanted some Japanese voices.

Q: Would you have done this even if it did have voices?
A: Yes I would've

Q: What voices are in it?
A: Subaru (Geo), Luna, Kizamaru (Zack), Gonta (Bud), Misora (Sonia), Solo, and I believe War Rock (Omega-Xis) at the title screen. Subaru's voices is also in-game when you transform into Tribe ON.

Q: Can I use Action Replay codes?
A: Unfortunately, yes you can. The Game ID will change though. So you need to figure that out before you can use cheats. All cheats that work with the Unhacked MMSF2 will work with the Hacked MMSF2.

Enjoy