OK, in the never-ending saga of Skype and Linux, here's how I got Skype (Beta) 184.108.40.206 to work with SL x86-64 3.4f (Skype was giving the error message "Call Failed: Problem with Audio Capture").
1. emerge -u to get the latest version of the ALSA packages alsa-firmware (1.0.14), alsa-headers (1.0.14), alsa-lib ((1.0.14a-r1), alsa-plugins ((1.0.14), alsa-tools (1.0.14).
2. Run alsaconf as root in a Terminal window and click on the OK buttons and click on your audio card. It should be obvious.
3. Run alsamixer as root in a Terminal window and make sure nothing is muted (pressing M toggles on/off) and volume levels are turned up (not into the red region, though). Ditto for the two Capture channels.
4. Run GNOME Volume Control or KDE KMix (KMix does not work properly on my laptop: Capture channels reset every time) and double-check nothing is muted and levels are all non-zero.
5. emerge -u to get the latest version of Skype (220.127.116.11).
6. K Menu > Control Centre > Sound & Multimedia > Sound System, click on the Hardware tab and tick the box 'Override device location' and enter "default" (without the quotes) in the box beside it. Don't change anything else in the as-installed settings. Click 'Apply'.
8. Launch Skype and select Options > Sound Devices. Make:
Sound In ---> HDA Intel (hw:Intel,2)
Sound Out ---> HDA Intel (hw:Intel,0)
Ringing ---> HDA Intel (hw:Intel,0)
N.B. My sound card is HDA Intel. Yours could be different, but try to select the (hw:*,2) and (hw:*,0) as I've shown above.
Note that this procedure is different to the procedure in the Wiki for earlier versions of SL and Skype.
Now why is it that, when I upgraded to the latest version of Skype for Windows under Windows XP this morning, it just worked?!
EDIT: If the above does not work, try also: K Menu > Control Centre > Sound & Multimedia > Sound System and untick 'Enable the sound system' under the General tab.