Have you tried Code 30186 from the DirecTV Site?
Model (optional): HT-SB40 Soundbar
We could not find an exact match, we think this code will work.
1.Set the "Mode Switch" to Device 3. [AV1 or AV2]
2.Press and hold the "Mute" button and the "Select" button until the visible LED blinks twice.
3.Enter the 5 digit Code [ 30186 ] above. [LED blinks twice]
4.Try your new code [press and release PWR key ... should turn Soundbar Off] and tell us how it worked!
NOTE: The VOL key will NOT WORK with the Soundbar unless you have previously Activated the VOL key by entering a TV Code into the Remote with the MODE Select switch set to TV.
If you enter the 30186 Code, with the MODE Select switch set to the AV1 or AV2 position, and the Remote's LED blinks twice... the Code is a valid Code for the Remote. If you then press and release the PWR key and the Soundbar turns off, then the Code may work with your Soundbar.
If the Code allows the PWR key to turn the Soundbar off, you may still have to LOCK the VOL key to operate with the Soundbar by entering the 993 Code and then pressing the Remote's SELECT key. The 993 Code allows the VOL key to control the Soundbar when the MODE switch is in the DTR [far left] position.
Also NOTE: You do not need a software update for the Code to work; because the Codes are part of the Remote's firmware NOT the Set-top Box.