In Arabic, such informative sentences (that is, sentences which include subject and predicative) don't start with an indefinite noun such as (a lawyer). There are just few exception to this according to strict rules. So, the general rule here is that the subject of the sentence must be definite (in nominal sentences like the case here). So, a translation to a lawyer is important would automatically be المحامي مهم (THE lawyer is important: al-muHámiy muhim).
I can say it exactly as you worded it in English, using "there": هناك محامٍ شريف (شريف šaríf here being equal to "honest" or "conscientious" - beside other meanings).
However, in the context you've just mentioned, I can imagine a scenario of two people arguing and the second person is "contrasting" the first statement and typically for emphasis, one also would add بل (bal) before the sentence I've just mentioned above, or I can say: بل يوجد محامٍ شريف (bal yújad muHámin šaríf). Here, the word (bal) is equal to the German (sondern) which I think it doesn't have a precise equivalent in English, but I see the German word here fits perfectly for the meaning of this word. And then (yújad) means (it exists). So technically, I'm saying: "but" there exists an honest lawyer.
Actually the audio is wrong. To be grammatically correct, it should be: مُحامٍ مهم (muHámin muhim). The word محامي (lawyer) is a special kind of word because it ends with a vowel (í) ي. When such words come without the definite article (AL الـ), the last letter is omitted and replaced with Tanwin.
It is Mubtada' indeed. However, it is not always possible to put Dhamma (-u) or (-un) at the end of the word. Typically in Arabic grammar books, in such cases they say ضمة مقدّرة (Dhamma Muqaddarah) meaning an estimated Dhammah. Because it should be there but the word's ending blocks that Dhamma for showing (for various reasons like the case here).
For example: عيسى وَلَدٌ ذَكِيْ (eesá waladun ðakiy); Meaning: Eesá (Jesus) is a smart boy.
Eesa: Mubtada' (subject).
Waladun: Khabar (predicative).
ðakiy: Cifah (adjective to Waladun).
Here, عيسى is a name that ends in Alif-Maqcúrah ألف مقصورة, that is ى (don't mix this with ي Y). This name is mubtada' indeed, but we cannot put Dhamma there because it ends with (-á).
muHámin is the same, we can't put Dhamma at the end because the noun itself has undergone a surgery - one vowel is removed and replaced with Tanwin.
Apologies, I was reading it a bit wrong here but anyway the main concept here does not change. The sentence محام مهم is actually a phrase composed of Noun+adjective only and not a complete sentence with subject (mubtada') and predicative (khabar). However, the rule of Dhammah and all that does not change - it's just essential to correct the understanding of the sentence.