I’m building a small Bluetooth speaker based on a board (DW-CT14+) that outputs two amplified audio signals (left and right stereo channels). I only have room for one speaker in my device so I need to combine these into one mono signal. How is this done?

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I tried wiring both outputs to the speaker through a pair of resistors, as many articles suggest. I tried 100 ohm resistors first but the sound was too low. I switched to 10 ohm, and the sound is loud enough, but every few seconds the sound glitches and the whole board seems to reset. I'm not sure what I'm doing wrong.