Can I configure my iPhone to play audio files through ear speaker instead of the bottom speaker? My company utilizes a voicemail system which sends audio files as email attachments. These audio files playback via QuickTime in iOS which pushes the audio out via the phone's speaker. Which is great for a Mumford and Sons song, not so great for voicemail. I can hook up headphones for private listening, but when I am without headphones wanted to use the phone's ear speaker instead.

Using an iPhone 5 and iOS 6 is it possible to play audio files played back via QuickTime with the ear speaker? If not via QuickTime is there an app for that?
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.