Most Heath Zenith product manuals are available here: https://www.heath-zenith.com/support/owners-manuals
. Searching for the SL-6273 found two versions of the wireless MP3 door chime. The direct link to the manual for the SL-6273-00 is https://www.heath-zenith.com/system/spree/documents/attachments/000/000/839/original/205371-02A.pdf?1436551434
Put the batteries in the door chime. To transfer an MP3 to the door chime, connect it to a computer with a USB cable. Open the folder to view files and drag the new (short, <= 20 seconds long) tune into the folder. Eject the door chime (like any USB hard drive) and remove the USB cable from the door chime.
Next press the Tune button to cycle through the tunes on the door chime for the front door chime. Stop when the desired tune is played. That will be played any time the front chime is pressed. If you add the jumper to pin 8, the door chime will ding as that is the rear rear door sound. For the front door, make sure that the switches (1-7) on the push button match those for the door chime.
I hope this helps.