Can't get sound from USB audio device

[Dave M G]

Ubuntu Users,

Recently my friend gave me an old computer of his, and I'm going to turn 
it into a dedicated MythTV media device.

But that's going to be tough without sound.

The only sound device connected to the machine is an external USB sound 
device (Kenwood). I've tested this device on my other computers and I've 
verified that it works.

When I open the Volume Control GUI and click File and then "Change 
Device", I can see that my "Kenwood USB Audio" device is the only device 
listed there.

When I open up Preferences-Sound, I can use either "Autodetect", or "USB 
Audio", and press the "Test" button. I get no errors, and it shows an 
indicator that something is playing, but no sound.

Even when I open a video file or MP3, I get no errors, whereas I would 
expect a "cannot connect to sound device" error, like I've seen before 
in similar circumstances.

And then, oddly enough, when I opened Gedit, I got a stream of errors 
that looked like this:

ALSA lib confmisc.c:670:(snd_func_card_driver) cannot find card '0'
ALSA lib conf.c:3500:(_snd_config_evaluate) function 
snd_func_card_driver returned error: No such device
ALSA lib confmisc.c:391:(snd_func_concat) error evaluating strings
ALSA lib conf.c:3500:(_snd_config_evaluate) function snd_func_concat 
returned error: No such device
ALSA lib confmisc.c:1070:(snd_func_refer) error evaluating name
ALSA lib conf.c:3500:(_snd_config_evaluate) function snd_func_refer 
returned error: No such device
ALSA lib conf.c:3968:(snd_config_expand) Evaluate error: No such device
ALSA lib pcm.c:2145:(snd_pcm_open_noupdate) Unknown PCM default
ALSA lib confmisc.c:670:(snd_func_card_driver) cannot find card '0'
ALSA lib conf.c:3500:(_snd_config_evaluate) function 
snd_func_card_driver returned error: No such device

So I hit the Google trail, and didn't see anything that I could 
understand which had concrete advice on what to do about it.

But I came across this web page:
http://ubuntuforums.org/showthread.php?t=402293

... which seemed to have some relevant information on editing the 
/etc/modprobe.d/alsa-base file.

By changing the first ltwo lines in that file to:
install sound-slot-0 /sbin/modprobe snd-usb_audio
#install sound-slot-1 /sbin/modprobe snd-card-1

Got rid of the errors above, but I still didn't have sound.

At this point I realized I was whacking away at a file that I didn't 
really understand, and thought I should turn to some experts to see if I 
was barking up the right tree.

Is this the right file to be editing to fix my audio woes? Is there 
anything else I should be checking to diagnose this problem?

Any advice or information much appreciated.

-- 
Dave M G
Ubuntu 7.04 Feisty Fawn
Kernel 2.6.20-15-generic
Pentium D Dual Core Processor