Since mud brick is a dense and rather solid material, its sound absorption is rather low, similar to that of  plaster, that is 20% at low frequencises and 30% at medium (500 Hz) and higher frequencies

Good afternoon

You may want to look into:

https://www.researchgate.net/publication/248541976_Sound_insulation_of_fibre_reinforced_mud_brick_walls

With best wishes

Vladimir Ladinski

Article Sound insulation of fibre reinforced mud brick walls

Dear Cristóbal

The following article is a good literature related to sound absorption on mud bricks (adobe):

Hope you find it helpful,

Bests- Farzaneh

http://www.sciencedirect.com/science/article/pii/S0950061808001220