The Thousand-Year Door has gathered less traction than its N64 counterpart, so there are lesser tools available for hacking the game. There are a few, though!
Download Wii.cs here.
Download the tool here.
Audacity (jackoalan's custom-fork)
A custom fork of Audacity maintained by jackoalan. It has the capability to open .stm, .dsp, as well as a few other proprietary audio formats. Can export saved work to .stm or .dsp as well.
Download it here.
Download link here.