Hi,

Is the sound normal when using a set of headphones plugged into the tablet?

If so check that the speaker grill on the back of the tablet is not covered, (by a case for instance) or if it is clogged. Do not poke anything into the grill, you may damage the speaker.

If it looks dusty/dirty try just a very quick suction action using a vacuum cleaner (on a low setting if possible) with an upholstery brush attachment and see if this improves the sound.