I'm surprised to see that we're discussing options again. Making the ability work a lot like Scorching Ray would provide built in balance against an existing and rather ubiquitous game element (Scorching Ray). I'd offer 3 rays spaced out over a few levels. You could go with 6/8/10, 6/9/12, or maybe even 3/7/11 if the first ray replaced the pointless Wordstrike. The final option would scale just like Scorching Ray. I personally prefer 1d8+Cha for the damage, and that shouldn't far exceed Scorching Ray's 4d6 unless the Bard's Charisma is well over 30 (in which case you'll have other problems with saving throw DCs and such)
The damage might seem a little low, but if you throw on Arcane Strike and Discordant Voice it should scale pretty well. Physical damage plus Clustered Shots would probably be stronger than sonic damage overall since the latter can easily be shut down with Resist Energy. That said, sonic is certainly thematic for a Sound Striker. It would be useful in a wide variety of fights without a heavy feat investment, and it might help appease concerns that the power would be too strong if it wasn't subject to energy resistance like Scorching Ray.