You can find various German pronunciation practices via Google, but I found German pronunciation practice useful to make a hand in German vowels.
For more information before clicking the link, it includes:
- Pronunciation of German vowels
- Pronunciation of "o" in German
- Pronunciation of "ö" in German
- Short or long vowels in German
- Double vowels' pronunciation in German
- Pronunciation of diphthongs in German
- Pronunciation of "r" following e, o, u, ä, ö, ü
- Pronouncing "e" in the last syllable
- Pronunciation of "ch" in German
And each part is completed by a good list of examples which can be played, you can see the translation of words by moving the cursor over the word.