I switched to a new host provider, and was no longer able to embed video tags - due to MIME problems. How solve?

I switched to a new host provider for my website, and I found that putting in a video tag into a webpage with webm type videos did not work.  Their tech support said I could add MIME types to my web.config, and that would solve the problem.  I did add some MIME types, but now I get an error message that my MP4 mime types can only have one association.  What is wrong with what I did here:
  <system.webServer> <staticContent> <mimeMap fileExtension=".mp4" mimeType="video/mp4" /> <mimeMap fileExtension=".m4a" mimeType="audio/mp4" /> <mimeMap fileExtension=".mp4" mimeType="audio/mp4" />
and of  course: <mimeMap fileExtension="webm" mimeType="video/webm" /> </staticContent> </system.webServer>
Thanks a bunch.