[Goblingil]

Howdy folks, I was looking for some advice for my first romhack project. As part of my mod, I'm looking to add voice clips to Link To The Past. When I first had my idea, I naively thought "oh, this will be easy, just swap out existing sound clips!".....LTTP doesn't use sound clips. it uses channel sounds. they added the voice clips in the GBA remake, and the hacking utilities for LTTP don't allow for dropping in sound clips. so I figure I need to do it the hard way, and hex edit my way to glory. but I was hoping to gauge just how difficult such an undertaking might be before I jump into it. I want to make sure I know what to focus on, or if what I want to do is even possible.

[optiroc]

Doing it as an MSU-1 hack would be rather easy. Going the “native” non-expanded cartridge route is definitely not trivial... Fitting speech in S-SMP RAM is tricky, especially if you don’t want to interrupt the regular music playback.

MSU-1 is probably your best bet.

[Goblingil]

could you elaborate some on what that means, or at least, how I can find out more about the process?

[Jorpho]

Think of the MSU-1 as a custom "chip" like the SuperFX.  It enables the Super NES to easily play sound samples.  (This is a a relatively recent development used mainly in emulators and some flash carts; such a thing would have been much too costly back in the 90's.)

Conker's High Rule Tail is a LttP hack that uses MSU-1.
https://www.romhacking.net/hacks/3346/

As for how exactly you would modify a ROM to support the MSU-1, I wouldn't know.  There is surely documentation out there, though.

[optiroc]

There’s source code for several MSU-1 hacks on GitHub, as well as other example code that shows how to interface with the MSU-1.

In short, the audio part of the MSU-1 works pretty much like a CD player. You write a track number to a register and tell the “chip” to seek. When ready, you tell it to play.

So you need to reverse engineer the sound fx code for the game, and replace certain calls to the normal S-SMP engine with MSU-1 calls.