Tell us about yourself!
Actually, ac-dc is right. I forgot you are using a wall wart. Those have there own filtering built in. A regulator would be the better option. Are you trying to make passive speakers into active or just trying to replace battery power for AC/DC? If trying to make yours speakers active, that's a whole other ballgame. You'll need test and stimulus equipment to characterize for frequency response, impedance matching, etc...
You would need to add a capacitor for filtering the noise.
Tom Kha Gai -- Thai Coconut Soup