This first is a video specifically for people who do voice over work.  It talks about the importance of developing accents for the current voice over market.  It also gives some tips for how to go about developing those accents:

These next two resources are both websites with extensive databases of audio clips.  The first is a bit more scientific.  The clips feature the same carefully selected paragraph which contains “practically all of the sounds of English”.  Many of the accents have a phonetic spelling out of the audio on the page with the clip.  It’s a treasure trove.  Check out the Speech Accent Archive.

This second site is less scientific, but it’s also a little more fun.  The standard paragraph that gets read is not so specifically chosen, but it is followed by a couple of minutes of the subject talking about some experiences in their life.  It’s more personal and free from.  This site was put together by a theatre professor for the purpose of studying accents.  Take a look at the International Dialects of English Archive.

I am going to start with a British accent and possibly a German one.  I’ll let you know how it goes.