Learn Italian: Il muro è sporco.

No, Italian has two common words for wall. Muro usually refers to an exterior wall, a brick or stone wall, or a structural wall. If you are talking about the interior walls of a room (like in a bedroom or living room), you would normally use the feminine word parete.

How would you make this sentence plural?

You would say I muri sono sporchi. However, muro is a tricky word because it also has a feminine plural form: le mura. You use le mura when talking specifically about the defensive outer walls of a city or a fortress. For regular walls, just stick to the regular masculine plural i muri.

Why does the plural of sporco change its spelling to sporchi?

Because sporco ends in -co, we have to be careful with its plural form. To keep the hard k sound that you hear in sporco, Italian requires you to add an h before the i in the plural. If you just wrote sporci, it would be pronounced with a soft ch sound like the start of the English word cheese.