Thanks, that works perfectly, I had to modify it slighty to do what I wanted:
tell application id "com.figure53.QLab.4" to tell front workspace
set cuesToProcess to cues of cue "CUENUMBER"
set selectedCount to count cuesToProcess
set i to 0
repeat until i = (count cuesToProcess)
set eachcue to item (i + 1) of cuesToProcess
if q type of eachcue is "Group" then
set cuesToProcess to cuesToProcess & cues of eachcue
end if
set i to i + 1
end repeat
repeat with eachcue in cuesToProcess
if q type of eachcue is in {"Audio", "Video"} then
eachcue setLevel row 0 column 3 db -120
eachcue setLevel row 0 column 4 db -120
end if
end repeat
end tell