Does this weapon ability sound right?

[KarinsDad]

My point was that it is in a base class ability.

Err, so? Most spell casting and psionic manifesting classes can get ER as well. That does not mean that players will often (if ever) take ER:Sonic with these.

And therefore is only as rare as that base class is in the setting. Asking how many 10th level warlock PCs or NPCs there are is a irelevent question - it can be a commmon class depending on the game being run. And since the common opinion on these boards seems to be that the warlock is a powerful class - I would tend to think there would be more 10th level warlocks than say 10th level barbarians.

It matters not if all 4 PCs are Warlocks, none of them (if played intelligently) should not take ER: Sonic unless the DM gives them reason to believe that Sonic attacks will show up in a frequency greater than 20%. Ditto for NPCs.

For example, Shout does 5D6 at 7th level whereas Cone of Cold does 7D6. At 10th level when the Warlock first gets his ER, it is 5D6 versus 10D6. Unless the DM likes to Deafen players a lot with spells like Shout, there is absolutely no valid reason to take ER:Sonic.

Your point appears to be moot because you are talking 2 base classes (Warlock and Favored Soul) out of 37 in the PHB, Complete books, and XPH who can get it as a class ability as opposed to a spell/psionic power.

Balance-wise, 2 base classes with an ability (and then not typically taken like ER:Sonic) out of 37 is absolute white noise.

Personally I would take ER sonic instead for the very specific reasons that you've pointed out that there are few ways to get resistance to it, other than silence. And since there are fewer ways to get that protection the protection is more valuable.

Sonic spells/powers are generally weaker and generally less frequent in the game. Virtually nobody takes a protection against a rare weak attack over a powerful frequent attack unless it is for RPing reasons or the DM gives the player a reason to believe that it is not as rare as normal. Since the main ways to get ER in the game for the vast majority of PCs is Resist Energy or Protection From Energy and these allow for any of the resistances, your reasoning here does not make sense.