You'll have to give a little more details how you have things connected because I don't understand where you're using the RCA cables.

But my best guess:

There are no RCA connections on Mackie PROFXv3 mixer. The main outputs are 1/4" but they can't use "speaker cables". "Speaker cables" usually means cables that are used to connect a powered mixer to unpowered speakers, which is the opposite of your setup. The outputs from the mixer would use cables labeled for "instrument", "guitar", "patch" which are used to connect your unpowered mixer to a powered speaker.

But in your setup I think you need a special cable. For cabling from the Mackie mixer to the soundbar I think you need a dual mono 6.5mm to stereo 3.5mm cable like this

https://www.amazon.com/NC-XQIN-Splitter-Compatible-Multimedia/dp/B08GSGXDZN/ref=sr_1_4?dchild=1&keywords=dual%2B6.5mm%2Bcable%2Bto%2Bstereo%2B3.5mm&qid=1610455785&sr=8-4&th=1

And if you decide to use a powered speaker like the Rockville (which I strongly recommend), the same cable can be used.