There are several types of reducer insulated crimp connectors that can be used for this. The one shown below is for #14-16 to #18-22. This will fit through holes that you might have to drill going from cabinet to cabinet. These will work for your 12 volt runs but don't even think about splicing your 120 volt runs. This crimping tool is inexpensive and can be used for many different connectors. You might have to get your plugs and add some #18 to 20 gauge wire to them and then splice to your #14. You also could remove some strands of your speaker wire so it fits into the plugs. You could never do this with any household 120/240 volt wiring but would be OK for the 12 volt led fixtures
