If you have a portable speaker laying around that uses AAA/AA batteries why not same the money and power it with USB
Well let's first note that this mod will properly only work if the voltage is correct. In my case the speaker needed 3x1.5V=4.5V AAA batteries, which happens to be pretty close to the 5V from a standart USB cable.
Remember anything you read on the so called ""Internet" is on your own responsibility.
You probably don't want to permanently solder the USB cable to the speaker so grab two empty batteries.
Usually the batteries are connected in a series so you only need the first and last connector. Test to find which once are the correct once, and then just solder the cables to the batteries and put the batteries in the speaker.
For added convenience if your speaker has a backplate that covers the batteries, just use your soldering iron to melt a USB sized hole in the backplate.
Do however note that if you die of the toxic fumes, that's on you.