Yesterday, following a mention I heard on the Linux Client Users group, I found the wrapper script which runs the Windows voice component in WINE (attached to http://jira.secondlife.com/browse/VWR-2041).
It partially worked for me - it's extremely slow to connect to the voice server (over a minute), and it loses its connection after a few seconds, requiring that voice be disabled and re-enabled in Preferences before it will successfully connect again - but I did actually manage to hear part of somebody's sentence.
But the rest of the text in that JIRA issue looks promising - it talks about running the voice component on a separate machine. My Windows laptop doesn't have enough power to run SL, but it might be able to handle the voice part.
SLVoice.exe displays its parameters if you pass it "/?" on the command line, but SLVoiceAgent.exe doesn't - does anyone know what its parameters are?
(Or would any Linden be kind enough to explain exactly what SLVoice and SLVoiceAgent do, and how to use them so that a Linux box can communicate with a Windows box running the voice component?)