Unfortunately the only way to get audio from a PSR-400 is to connect to the headphone jack or external speaker jack. I get best results connecting to the EXT SP
The next step is to set the audio input level to your computer. I'd set the volume control on the scanner to about "11 o'clock" and then adjust the input level on the computer while monitoring the level with software that displays the level. I use this digital level meter
I would not run the volume of the scanner past 1 o'clock, and you might have to run it lower than 11 o'clock to get good control. It has to do with your sound card more than anything else. Some sound cards provide an automatic level under the advanced control, you'll have to look there. But I find even when using the automatic level, I still need to adjust the manual setting.