Short answer, no. Unity (because it is a heart a 3D engine) compresses everything to hardware-optimized compressed textures (DXTx, or other platform specific versions). And that requires the dev to do something, it's not something you can easily mod, like with RenPy.
I know this is kinda old but I thought I might share what I learned when trying to compress unity games.
There are some unity games that are 100MB inside a .rar and turns into 2GB after extraction. My main focus is was to reduce the size of those game cases.
After extracting the files the reason for that size difference is basically as stated above. All image files and audio files are not compressed at all making a single music 40MB and a big image 7MB while the game might have almost a thousand of those.
I did not have any luck puting the audio files back onto unity but I had some mild success in putting the compressed image files.
There were 2 ways that I found it possible to do that. Here are them:
The first way was using "Unity assets bundle extractor"....
This tool can extract texture files as .png or .tga files but even if you compress the .png when you insert them back it'll turn back to the uncompressed size to go with unitys texture format.
But if you click a single Texture2D and use the Plugins->Edit... you can change the Texture format and the tool will compress it properly.
You don't get any options but you get a fair choice of types. Just changing the texture from ARGB32 to DXT1Crunched(slow!) for example made the image 50% smaller.
This was just a test to see if it was at all possible.
I could not find any way to batch convert the textures and when saving the file back the new image are put together with the resources.assets instead of the resources.assets.resS file.
So you'd need to convert all the textures2D and them delete the now unused resources.assets.resS(which only had the texture files).
Audio clips are stored on sharedassets0.assets.resS.
The other more proper way:
Using UnityEX you can extract all Unity's Texture2D as .tex files and reimport them back.
So if you could convert the .tex files to a proper more compressed format and reinsert them back it should work.
I could not test that as I had no idea and could not find an easy-to-use tool to convert textues .tex format.
Disclaimer Notes:
Reinserting AudioClips back into unity is way too complicated so I give up on that.
I don't know anything about compression and only tried to use the already made tools to solve this problem so I might be wrong.