Translation:The car is dirty.
Some people pronounce things wrong lol 더 at the beginning of a sound is pronounced like the "taught" in "taught"
I'm pretty much sure the pronunciation of Duolingo (especially for Korean) is really bad. So you should ignore when it sounds odd, and follow the standard pronunciation. In this case, ㅓ sounds like 'eo', so stick to that.
The correct pronunciation is [더럽씀니다]. The TTS's pronunciation sounds okay to me.
Same here, and there have been a few like that as well. My wife (Korean) said that several of these are pronounced slightly wrong, so I'll go with your second one and assume the spelling is authoritative instead of the recording.
It sounds simillar to an "A" sound because it's an open "O" sound. Like the "ou" in "sound"
더 at the beginning of a sound is pronounced like "taugh" in "taught" or "ta" in "talk"
The Korean sentence is using relatively formal language; contractions like that are too casual to be an accurate translation.