Audio Cable Connection (Audio In Port, Portable Audio Device) - Sony SRS-X99 Help Manual

You can connect a USB flash drive to the USB A port.
USB connection (USB A port, Walkman)
You can connect a Walkman to the USB A port.
USB connection (USB B port, High-resolution supported Walkman)
You can connect a high-resolution supported Walkman to the USB B port.
USB connection (USB B port, PC/Mac)
You can play High-Resolution Audio format files on your PC with the speaker, after downloading and installing
the required driver and Hi-Res Audio Player to your PC.

Audio cable connection (AUDIO IN port, portable audio device)

You can connect the device to the AUDIO IN jack with a commercially-available audio cable.
How to Use
Getting Started
SongPal
Listening to music via home network
Listening to music via BLUETOOTH connection
Listening to music via USB connection
Listening to music by connecting with an audio cable
Listening to music on an Xperia
Listening to music on a smartphone