You should be able to access the A/V input by cycling through all available channels (using the Channel Up/Down buttons). You may need the A/V device to be powered on. However, if you programmed the Channel list to skip Aux, you may need to add that back into the list. You need the Menu function on the remote. Use the Up/Down arrows to navigate to Channel Set Up. Then press the F.Fwd (right arrow) to enter the sub-menu. Then select the Add/Delete Channel option.
Another way to access the A/V input is accessed by pressing the Game button on the original remote. (This also adjusts the screen lighting.) The game button is the middle button on the last row of buttons on the remote. If you do not have the original remote, you need one that matches that function. Here is one that is guaranteed to work: http://remotes.com/store/symphonic/sc3813.html
. This one may also work: http://www.emtcompany.com/symphonic-sc3813-remote/rc64.html
The Symphonic SC3813 was manufactured by Funai. The manual is available from Funai service support: http://www.funaiservice.com/manuals/SYMPHONIC/SC3813/
. The direct link to the manual is http://www.funaiservice.com/manuals/SYMPHONIC/SC3813/SC3813.pdf
I hope this helps.