Media volume suddenly became too low
Finally after a week, i found the answer.

The CyanogenMod AudioFX was conflicting with the music app. In the previous version od CM, the AudioFX could only be turned on if a headset or speaker is plugged in. But in the recent update, we can activate AudioFX for the Phone's speaker too. Thus, in my case the AudioFX was turned on (I was not aware of the update)

But the HW+ decoder of MX player overrides AudioFX, thus I was able to hear the sound while using MX player.