Subsonics

(Complete Adventurer, page 112, DnD 3.5)

[General]

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

Prerequisite

Perform (any) 10 ranks, bardic music

Benefit

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