Subsonics

(Song and Silence: A Guidebook to Bards and Rogues, page 40, DnD 3.0)

[General]

Your music can affect even those who do not consciously hear it.

Prerequisite

Perform 10 ranks, Bardic music ability

Benefit

You can play so softly that opponents do not notice it, yet your allies still gain all the usual benefi ts from your bardic music. Similarly, you can affect opponents within range with your music, and unless they can see you performing or have some other means of discovering it, they cannot determine the source of the effect.