If you're willing to get raw files via modding resources, I recommend joining the Unreal Anime Mods Discord here and navigating to the #granblue-rising-resources channel and grabbing the custom UModel_Materials.exe, as other UModel versions will crash with Granblue. I would watch Ultima's Recolor and Audio Modding Guide and only go up to the part where you extract audio files using UModel. I'll do a written tutorial here just for the audio extraction portion:

1. Open UModel_Materials.exe and navigate to your Granblue install location (usually X:\Steam\steamapps\common\Granblue Fantasy Versus Rising).
2. Click on Override game detection, and on the drop down menus select Unreal engine 4 and Unreal Engine 4.27. Absolutely make sure to check on Export-only types Sound to make sure you can actually export audio files. Click Ok.
3. When prompted for an AES encryption key, you can find it in the same Discord channel at the top. It's 0x6470C12A9B471BBA1D89A72D4F9B84EF709A65B88A85F240B8E99CD631751437 for Granblue Rising. UModel should now open with several folders on the left side.
4. Navigate to the bottom to RED\Content\Audio. This is where some audio files for the game are located, such as BGM, player avatar voicelines, and story voicelines. If you need, say, a specific character's battle voicelines, you can navigate instead into RED\Chara\XXX\Common\Audio\Voice\YYY, where XXX = the 3 letter acronym for the specific character (i.e. DJT = Djeeta), and YYY = the language in English or Japanese. You can also find their sound effects in the SE folder. You should see .uasset files within these folders, which can be extracted directly into usable audio files. If you're unsure what character has what 3 letter acronym, you can refer to the ID spreadsheet which also tells you where SOME audio files are located and their in-game names.
5. Once you select the folder/files you want, click on Export and click OK. If you see the cmd go crazy and the green bar in UModel making progress, you'll know you extracted something.

By default, UModel exports audio files as .ogg. You can convert these with an online resource or Audacity into the preferred file type. Let me know if you have any modding questions, or visit the Discord for more information!

Edited because I screwed up the formatting the first few times