Can I configure my iPhone to play audio files through ear speaker instead of the bottom speaker?
No you can't.
And as far as I can tell (searching the App Store) there are no apps that can help you. Even if there was one you wouldn't be able to open the audio file in Mail without using QuickTime, and that will always lead to the audio file playing through speaker.
Best you can do is have a pair of headphones with you and hope something changes in the future.