On a Windows 8.1 device that supports InstantGo, you can play music through a Microsoft Store app such as Xbox Music, and the audio will continue to play even when the device goes into standby mode. If the device has physical volume buttons on it, you should be able to adjust the volume even when the device is playing audio in standby. However, if you have Chinese, Japanese, or Korean Input Method Editors (IMEs) installed, volume buttons wont work in standby.
The IME process doesnt work correctly with InstantGo standby mode.
To work around this issue, bring the device out of standby mode, and then adjust the volume.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.