Their dog is barking / barks so loudly? I can't stand it.

their dog is barking barks so loudly? I can't stand it. Их собака так громко лает (именно сейчас в данный момент) Я этого не выношу.

their dog barks so loudly? I can't stand it. Их собака так громко гавкает (вообще, постоянно).

Все зависит от текста!

Their dog is barkingso loudly. I can't stand it. - Present Continuous

Their dog barks so loudly. I can't stand it. - Present Simple