For the voices in the game, I use .ogg files. I think you can get away with using .wav or even .mp3, but if you want to be extra sure, follow all the steps detailed here.
First, let's get some generic voices from the internet. Freesound.org is an awesome resource for free sounds. This one will do.
Audacity is a very good free audio editor. I use it for minor audio editing all the time. Way faster than a proper digital audio workstation.
So, open up your audio editing software of choice. I'm sticking with an old version of Audacity here. Drag and drop your raw audio with the voice lines you want to add to the game.
Normally, I apply noise reduction, amplification and compression to get the loudest and cleanliest audio possible.
In Audacity, you can select any part of your audio clip and export it. For the voice lines in Orchid Rain, you need to have each voice line as its own audio file. In the case of this file, you'd need to grab every segment of audio and export it as its own file.
Name your files properly, as you might end up with quite a lot of voice clips. Ogg is a compressed audio format that comes by default with Audacity. For mp3, you need to download a plugin.
There are many types of voice lines in the game. For every type of voice line, an NPC will have many different versions to choose from at random when needed. A good mod will have many alternative versions of the same line, for all the lines it's replacing.
For example, a type of voice line is "aStartled". The A here stands for "alive", as it's a voice line that will be emitted when the NPC is alive (D sands for "death" and S stands for "sex"). NPCs in the game have around 3 different lines for "aStartled". An NPC has a chance to call this line when they spot the player. If the call is successful, they will choose at random one of their three different lines. If they only had one line, every NPC would end up saying the same thing all the time, and it would get old very fast.
Now, in addition to this, Doru NPCs can have one of three personalities. These are Normal, Sensual and Shy. When you set you voice clips, you will have the option to set different clips for each personality type, or just have one set of clips for all personalities. This will add more variety to the game in the form of animations and voice acting/lines as development advances.
Let's create a new folder, so we don't end up with tons of different files everywhere. I'll name mine "Audio". Double click to enter the folder.
Drag and drop all your audio clips to your newly created folder. Now, right click somewhere empty inside that folder, and go to Create > Game Stats > NPC Audio Clips. You will create a file that will let you organize all your audio clips.
The NpcAudioClips file will this when viewed in the inspector. "Replace missing lines with vanilla" will make the game play the original voice lines the modded NPC was supposed to play if the mod is missing the proper voice line. If you leave that unchecked, missing voice lines will simply not play.
Then, you have three property fields that are exactly the same. There is where you set the different versions of your voice lines according to personality type. You can leave the last two untouched. As long as you fill the Generic Voice Clips with your voice clips, the other two personality types will play the Generic ones.
When opening Generic Voice Clips, you'll see a "Size" field with a 0 written on it. Instead of 0, type 1 for now.
Two new fields will appear. Type will let you select which voice clip type you want to replace. There are many types of clips that the game still doesn't use, but that will be implemented as development moves forward.
Here are the sounds the game uses right now.
I've selected to replace aStartled voice lines. I want to use the 8 different voice lines I edited. So instead of dragging and dropping one by one, I'll do a little trick. While having the NpcAudioClips object selected, click on the little lock on the top-right of the inspector. This will lock the Inspector in whatever you had selected, freeing you to select other objects without losing the ability to "inspect" what you had selected previously.
Now, select all your voice clip files, and drag them right in the label that says "Clips". If you do it right, the Size field will change to 8 (or whatever amount of clips you have), and it will auto-populate with your clips. You can then unlock the Inspector.
You can add more elements to "Generic Voice Clips" by replacing the 1 in its Size field with 2, or whatever number you need. Repeat this process as long as you need to.
In my case, I only want to replace the aStartled voice line, and I want the rest of the voice lines to remain unchanged, so I've checked "Replace missing lines with vanilla". If you want the NPCs to not say anything other than the lines you are giving them, leave that field unchecked.
Select the child GameObject of ModelContainer and drag your NpcAudioClips file into the Audio Clips field, under "Other". After doing that, save your prefab, as we saw in the previous page, and build your mod.