Headphone, or line output cables typically have a signal level on the order of 1 VAC peak to peak (when the volume is turned up appropriately).
You need to offset that signal with a voltage divider, and make sure that any inherent DC offset is removed.
It appears from the photo that this is what you tried, but there is NO CONNECTION shown from the 10K/10K voltage divider to the audio input. The gap in the protoboard is an insulator.
Example schematic:
