I've had this problem with the visual voicemail app before and after the update. It has to do with the Hero switching audio outputs for some reason while in the voicemail app.
Here are the steps I use to reprodouce this problem on my Hero:
1. Plug in your headphones into the audio jack.
2. Go to the voicemail app and play one of your messages. It should play through the headphones as expected.
3. Exit the voicemail app and go back to the homescreen by pressing either the home or back keys.
4. Go into an app that plays sound (the music player or the voicemail app for example).
5. When you play the sound, the sound will come out of the internal earpeice of the Hero and not your headphones as expected. Even the music will play out of the earpiece instead of the back speakerphone.
6. To restore normal audio output, you must unplug your headphones. You can then plug them back in if you want.
A similar thing happens when I try to go into speakerphone when listening to messages. When you switch back from the speakerphone, the message will play from the earpiece instead of the headphones. And if I recall correctly, I've had similar behavior happen during voice calls, but I haven't been able to test that yet.
I'm wondering if others have this problem. If it's widespread, it could be a problem for those of us who often have headphones plugged into their Heroes. I just ordered some accessories to use my Hero in my car, and I plan to plug the phone into the aux port in my car stereo via a 3.5mm audio cable in order to listen to music/TeleNav directons/voicemail/phone calls.
I was able to replicate this same issue...had my phone connected to the car stereo via aux cable, listening to music, went to voicemail and listened to voicemail through the car stereo (via headphone jack) and when I exited the voicemail app, the music started up again, but it was going through the earpiece, not throught the headphone jack/aux cable to the stereo.
Mine works fine. I listened to a voicemail through an external source connected via 3.5 jack via the stock voicemail app. Then i exited and started a song from the stock music app and it played through my external source(headset).
Make sure you have an updated Voicemail app, mine was updated about a month ago. After opening the voicemail app, hit the menu button -> Settings and click on the Check for upgrade selection. My version is VN.184.108.40.206 and has options for playing through the speakerphone. Let me know if this helps.
I noticed the same thing also about a week ago. I plugged in my earphones to listen to music but i noticed i had a unheard voicemail. I went into the voicemail app and the funny thing is that I actually had sound coming from the earphones for a few seconds, then it stopped and was coming from the ear speaker on the phone. I didnt think anything of it since its not a very big deal to me. So after reading this thread I tried it again and the same thing happened. So it definitely isnt just your phone.
So has anyone figured this out or will we have to wait and see if 2.1 fixes it?
I don't often use headphones, but its driving a friend of mine who streams Pandora through speakers crazy! And I can't find a setting or app that will allow one to manually select the output device.