Hi MMC,
The sync cable is specially wired, so a regular 3.5mm minijack cable won’t work. However, this is rarely needed.
Its aim is to keep the speakers at a matched volume level when using the line input, but they won’t normally require this unless one speaker is hidden out of sight from the remote control (and I don’t advise this anyway as it would spoil the sound!)
If you really want a Sync cable then I can make one for you via my sponsor link below, but they are rarely needed.
Kind regards, Steve.