Always more to find
Soundproof's interaction with Heal Bell is implemented in Heal Bell, so it's inheriting Gen 5's code which includes the description. The only difference is Abilities still work while the Pokemon is inactive in Gen 3 and 4, and starting in Gen 5 are only effective if the Pokemon is active. I assume Game Freak made a mistake implementing the specific targeting mechanism used by Aromatherapy and Heal Bell in Gen 5, because they don't interact at all with Sap Sipper or Soundproof, respectively. Starting in Gen 6, they again correctly block the effect if the Sap Sipper or Soundproof Pokemon is active, but let it be cured if inactive. This is all mentioned in the Smogdex Heal Bell descriptions as well. I'll add a Soundproof description to Gen 4 on PS.