It isn't a hardware issue. Keep in mind that this is a tablet with stereo audio, not an entertainment center or a boombox. The speakers are capable of respectable listening levels in a small room if you are willing to sacrifice battery charge duration.
Asus has provided an app that can adjust the sound quality and loudness, called the Audio Wizard. If it is turned off, or is set to Voice instead of another mode, the sound may be lower than many music listeners prefer. The default setting is to provide maximum battery performance and sound quality with earphones.
You will find the Audio Wizard in in the Quick Settings panel (drag down from the upper right corner) on the top row, and also in the Drawer. If you rooted your tablet and deleted it, a version of the APK can be found on line.