I have used an external USB sound card for years in different forms. The recipe below is what I used to use, These days I simply use a puppy with pulse audio which makes picking up the external source easy, or I install pupseaudio to fossapup64.
But to stick with ALSA...
There is a particular issue to overcome in getting the audio recognized by the hardware, and a configuration file needs to be edited.
1) Click on the Multiple Sound Card Wizard, and select the sound card which is to be the default device.
2) NOTE the Card / Device settings - in my case it is Card 2: Pro SB X-Fi, Device 0.
3) Using Leafpad or Geany open the alsa.config file, found in /user/share/alsa.
Add/Change the following pairs of lines as follows:
defaults.ctl.card 2
defaults.ctl.device 0
defaults.pcm.card 2
defaults.pcm.device 0
defaults.rawmidi.card 2
defaults.rawmidi.device 0
defaults.hwdep.card 2
defaults.hwdep.device 0
defaults.timer.card 2
defaults.timer.device 0
Save the changes by clicking on the SAVE button on the top menu.
4) You also need to check that the configuration is correct for the Hardware, by opening up the configuration files for Retrovol and selecting Config Window. Ensure the numbered sound card matches what was noted in step 2: eg for Card 2 it would look like hw:2.
Click on Apply.
Hopefully the sound input/output will then be available through the external Sound Card.