Audio comes to Channel 1.
Some files, such as Apple Music tracks, can only be played with certain players, so you may not be able to play tracks like these.
Depending on the situation, you may need to change the audio setting on your PC/Mac.
Change the setting as follows:
Click the speaker icon in the lower-right corner of the screen, then click [Speaker] to switch to [LINE DJM-A9].
When using Windows 8.1 or earlier, set [DJM-A9] as the default device referring to [1. Check the default device selected in the [Sound] panel] in Case 3 of the troubleshooting guide [here].
*The guide is for rekordbox users. However, you can fix the problem in the same way.
Open [System Preferences] -> [Sound] -> [Output] -> [Sound Output Device] and select [DJM-A9] as the sound device.