I want to configure 2x/3x headset buttons presses. How can I do it?

Install any headset controller application. Here are some of them: JAYS Headset Control, Headset Droid

Why my headset buttons does not work?

Android has an weak media button management framework. It supposes that different applications may respond to media keys, but each application may provide own priority for such responding. Only applications with the priority high enough are receiving media button events. In case several applications has equal priority - they may receive events based on which application is running. But what to do when player should run on pressing the button? In case it is not currently running and there is some application with the same priority, the player will not receive the event. That is why a lot of players are boosting their priorities above allowed by the SDK. However they are still competing with each other unpredictably in case several players are installed.

Different headset management controls (Headset droid, JAYS Headset Control, etc) are using the same method. However, the advantage of standalone headset control applications is that they are configurable and may correctly redirect events to appropriate application.

There is rather different approach in mMusic. It has priority 0 (the same as standard android player). It supposes that the other installed applications are gentle as well and did not boost their priorities. In case it is not, or you want advanced features (like starting player on "play" button press or exotic actions on double-, triple-, etc presses), you free to use an standalone headset control application with boosted priority. mMusic will not compete with it. mMusic will listen to it instead.

Now, you can resolve your use case with either of two possible ways:

1. (Not recommended!) Uninstall all application with boosted priority on MEDIA_BUTTON receiver. Once it is done, mMusic will be able to receive media events.

2. (Recommended) Install appropriate headset controller application, which can capture MEDIA_BUTTON events, and redirect them to the chosen application. Please, note, that some headset control applications are specifically designed for single button headsets and will not react on next/previous button events.

I want to control player with volume +/- buttons to switch tracks. How can I do it?

Install appropriate controller application like Volume to Next Track or similar.

Why my SD card content is not visible in the file browser?

Go to Menu->Settings and change Root folder path to appropriate one.