Chosen Solution
I use my EVO 4g as an MP3 player in my car. I plugged the MP3 jack into the headphone jack yesterday and the music said it was playing but I heard no sound coming thru my speakers. Yes, it was plugged in all the way in the phone AND into the stereo. Is this something that I should troubleshoot or simply have replaced?
It is my opinion that if your headphones work in the same jack, but it does not work when you plug it into the audio in of your vehicle, that your jack is okay. Check the cable to your vehicle as well as the settings on, I assume, your car stereo. Does not sound like a phone issue to me. You might want to try another device on your stereo to make sure that it is working. Good Luck.
