You will no longer be using Dolphin after you get a n64 file. Dolphin collector runs a n64onwii in a wiionpc emulator, which is slower than Project64 etc.
What you're looking for is n64 custom textures (look for a n64 plugin with texture support) and custom music (can't help you with that, I suggest using cheats to disable music, along with external MP3/etc music)
If you just don't extract the rom and play straight in dolphin, you get custom textures out-of-the-box-ish, but still no custom music, as I don't think anyone has modified the emulator to have music (even less likely than a straight ROMhack)
What you're looking for is n64 custom textures (look for a n64 plugin with texture support) and custom music (can't help you with that, I suggest using cheats to disable music, along with external MP3/etc music)
If you just don't extract the rom and play straight in dolphin, you get custom textures out-of-the-box-ish, but still no custom music, as I don't think anyone has modified the emulator to have music (even less likely than a straight ROMhack)