Hi Stephen,
First, congrats for jumping on the DCC + sound bandwagon for 2-rail O Scale . Your choice of decoder is also a good one (very smooth motor control and great sound – provided you used a decent sized speaker and enclosure [make as air tight as possible for good bass]). For truck pickup feeder wires, #24 or #22 gauge stranded wire will suffice – I would lean towards #24 to not restrict truck movement. The Pittman can motors in those units do not draw a lot of current so #24 should be fine for the current handling needs over the short run between the truck and the decoder (even in the high current short circuit case). I would also recommend using flexible wire (more strand count for a given wire gauge) if you can find it. NWSL sells some real nice flexible stranded wired but the largest they stock is #28 gauge, so unfortunately that is a little too small for the larger O Scale needs. Maybe something is available on eBay but if you can’t find any, that is not the end of the world, regular stranded wired will do okay.
For the LEDs, #30 or #36 stranded wire gauge will be fine. LEDs draw very little current so you can pretty much use as small of a wire as you want without issue.
For the speaker wire, you can also use #24 or # 26 gauge wire (stranded is fine here too) but super flex wire is not needed.
Have fun and share a video of your new loco in action .
Scott K.
Austin, TX