Curse of Silence should tax the cursed player (2) whenever they cast the named card. However, when you bounce the named card from the field to the player's hand, Curse of Silence tax doesn't look like it's proccing. This is occurring in Brawl.
Steps to reproduce:

1. Target opponent with Curse of Silence and name their commander. If the commander has not been cast yet, observe the Mana Value reflecting the (2) tax. For example, the first time casting Bristly Bill, Spine Sower would cost 2GG instead of GG. The tax is permanent for as long as Curse of Silence is on the field targeting the player, so if Bristly Bill returns to the commander zone, the second time casting him will cost 4GG . +2 for commander tax and +2 for Curse of Silence.
2. Target opponent either plays their commander or already has the commander on the field.
3. Play a spell that returns the commander to their hand, such as Into the Roil or Bottomless Pool.
4. Observe the state of the named card. Ordinarily, with Curse of Silence on the field, the named card will show an additional (2) cost added. When bounced to hand, it does not reflect the tax.
5. Observe the opponent play the card at cost, without Curse of Silence tax.