Sprint HTC Hero :: Google Voice Voice Mails Only Play Through Speaker Phone?

I finally changed the voicemail system on my Hero from Sprint's "visual voicemail" (which is misleading because it doesn't transcribe voicemails) to Google Voice. It's nice to see my transcribed voicemails now. However, when I actually want to hear them (if the transcription didn't work well), it only comes through the external speaker; the speaker on the back of the phone. I went into Google Voice settings and under "Voicemail Playback" I tried the only two options: "Play via Handset" and "Play via Speaker." Unfortunately, both play through the speaker phone speaker. How do I change it to the phone speaker? I also can't adjust the volume, so the whole world hears my voicemails through the external speaker.