Playing audio through the earpiece speaker?

I receive .wav audio files in my email when I receive emails at my landline phone in my office. When I select these in the email tool, it downloads and plays the audio via quicktime through the speakers. For whatever reason, I am unable to play this audio file through the earpiece speaker (i.e. the "private" speaker). On voice memos and in normal voice mail I can enable and disable the speaker, but this does not seem to be an option when I play these files. Any thoughts? Thanks.