Solved! What cable should I use to connect one JBL 305 to a PC (3.5mm)?

Solution
The cable you link to is stereo at both ends. The plug at the speaker end is the correct type but not wired for your application.
The 3.5mm jack of the PC is unbalanced stereo.
The 1/4" TRS jack on the speaker is balanced mono.
You need one of the these
https://www.rdlnet.com/product.php?page=164
or you can make something similar
https://www.epanorama.net/circuits/linesum.html
The cable you link to is stereo at both ends. The plug at the speaker end is the correct type but not wired for your application.
The 3.5mm jack of the PC is unbalanced stereo.
The 1/4" TRS jack on the speaker is balanced mono.
You need one of the these
https://www.rdlnet.com/product.php?page=164
or you can make something similar
https://www.epanorama.net/circuits/linesum.html
 
Solution