I have a pair of JBL Duet speakers and 1 doesn't work unless the volume is maxed out on the other.

you can.

How to Connect JBL Flip 4 to Alexa Speaker - Tom's Tek ...

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Jun 29, 2021 - Connect JBL Flip 4 to Alexa Instructions, Step by Step · 1. Find the Alexa App on your Mobile Device · 2. Run the Alexa App · 3. Tap the Hamburger ...

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  How to Pair JBL Flip 4 with Amazon Echo Dot Speakers

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  Jul 9, 2021 - How to Pair JBL Flip 4 with Amazon Echo Dot Speakers · 1. Find the Alexa App on your Mobile Device · 2. Run the Alexa App · 3. Tap the Hamburger ...

Try doing default factory settings.
Step 1. Press the "Power" button located on the top center part of your JBL Charge 4 Bluetooth speaker. The speaker will be powered on when the LED backlight under the Power button is lit.
Step 2. Hold down the "Bluetooth" and "Volume Up" buttons at the same time for roughly three seconds. The Volume Up button looks like a plus sign. Additionally, it's important to hold down the "Bluetooth" button in the center of the device next to the Power button, as it's easier to hold down. After three seconds, the LED ring around the Power and Bluetooth buttons will briefly light up before the JBL Charge 4 powers down.
Step 3. Press the "Power" button again to turn the JBL Charge 4 Bluetooth speaker back on. The Charge 4 will instantly turn on in Discovery mode, with the Bluetooth LED blinking on and off, and the speaker making a pinging sound. This confirms that you've successfully reset your JBL Charge 4 to its default factory settings. You'll need to pair or re-pair any devices you want to play audio through the JBL Charge 4 speakers with the device.

Be sure JBL is charged, press the power button on JBL, press the Bluetooth button on JBL. Turn on your phones bluetooth,make it visible to all devices, make it search for aplliances. When it finds JBL - pair it (you may have to make up a pairin number). The "sun" button controls different sound qualities. And turn the volume up! You should download the JBL MUSIC FLOW APP from the APP STORE to make the most of it.

First question: which speaker is defective? The one with three cables + volume control, or the passive one with just one cable?

Because This Speaker is PASSIVE Speaker, and must Use Some Mini Audio Amplifier like 3-10 Watt to Drive the Speaker.

Make sure you are driving this with a balanced input XLR or TRS type cable. The driving device should have the SAME type output...

verify whit hear phones if is correct audio the problem is not in the computer...also check instalation driver sound is sound max for your computer

I have a pair of Media 3 JBL computer speakers. I am using a generic 12V DC adapter rated at 1000mA with a positive tip polarity, and it works fine.

Switch the speakers over just to make certain it's not the amp. If still bad on the good channel, then the crossover (inside) might have developed a fault.

If you have another computer or a friend, test them on his/her computer. Try this, as its fixed such a problem for me in the past. Start>All Programs>Accessories>Entertainment>Volume Control Then check the ticked/unticked boxes