Isso é o que eu encontrei quando pesquisei sobre este problema, espero que isso funcione para você, embora essa edição do registro seja para o Windows Media Player 10:
I checked my other machine and found the same behavior described above when previously it did not autoplay a CD when Media Player 10 was open. So I poked around the registry and found that the CDAutoPlay key was set to 1 on both machines under [HKEY_CURRENT_USER\Software\Microsoft\MediaPlayer\Preferences]. I changed CDAutoPlay to 0 and things are as they were before. Hope this helps others users having the same issue.
Caso você tenha que adicioná-lo, CDAutoPlay
é um valor DWORD de 32 bits.