Select all the items you want to change, then open the item properties, holding down Control key will allow you to fine adjust the volume fader.
Or else you could try this Lua script. Created long time ago from various forum posts, if item has multiple takes then I think I'd have to update it to loop through each take also, otherwise seems to work OK on selected items...
Code:
local vol_dB = -5.6 -- set item volume
reaper.PreventUIRefresh(1)
for i = 0, reaper.CountSelectedMediaItems(0)-1 do
local item = reaper.GetSelectedMediaItem(0,i)
local take = reaper.GetActiveTake(item)
if take then
local vol = 10^(vol_dB/20)
reaper.SetMediaItemTakeInfo_Value(take, "D_VOL", vol)
end
end
reaper.UpdateArrange()
reaper.PreventUIRefresh(-1)