Yes, by adding command(s) in the Activity Start/Exit feature which becomes available when you select "Yes, but I want to add more control of options and devices for this Activity" in the Review settings for this Activity. Keep pressing next/save until you get to that feature. Since using a pile of volume down commands is not very elegant nor efficient, you need to find a short sequence of command(s) to disable the TV speaker. Some TVs can achieve that with a single command like "SpeakerToggle" even though that command may not be available on the original remote. Thus you should try all the commands in device mode first.