You don’t have to worry about dialect as you should before long be able to make out those sounds, if you practice; once you master them as they are pronounced in the most accepted manner you will be able to recognize distortions without problem.

Here are two videos providing exercise for the nasals.

                              an-on
                              an-on-om

Here is some practice with é, è and e.

                              e-é-è

Practice with nasals "on", "un", and "en"

                              “ON”, “UN” et “EN”

Practice with [ø] (eu, jeu) and [œ] (eu, jeune)

                              ø et œ

Practice with "ou" and "eu"

                              « ou » et « eu »

Here is an excellent reference you should not neglect if your personal library suffers from a scarcity of books on pronunciation:
                               Phonétique et
prononciation du français
pour apprenants finnophones – Kalmbach.
The fact that it is useful more particularly to Finnish students of French is no hindrance to other foreign students. It is a thorough reference.

By doing some research on the web you will find other sources, and some, quite possibly better than those shown above; the list I provided is by no means exhaustive.