I discovered this by chance / luck.
Steam is configured to use ALSA Default for the mic with voice chat. Pulseaudio is configured to not automatically start or restart on my system. Pulseaudio does not block the ALSA Default audio pipe on my computer.

1 Start a terminal.
2 Start Steam separate from the terminal.
3 In the terminal start pulseaudio with 'pulseaudio -D' if it is not already running
4 In Steam goto the voice tab of settings, hit 'Detect Audio Devices'
5 In the terminal stop pulseaudio by running 'pulseaudio -k'
6 Click 'Test Microphone' and experience the crash.

Processor Information:
    Vendor:  AuthenticAMD
    Speed: 3600 Mhz
    8 logical processors
    8 physical processors
    HyperThreading:  Unsupported
    FCMOV:  Supported
    SSE2:  Supported
    SSE3:  Supported
    SSSE3:  Supported
    SSE4a:  Supported
    SSE41:  Supported
    SSE42:  Supported

Network Information:
    Network Speed:  

Operating System Version:
    "Gentoo Base System release 2.1" (64 bit)
    Kernel Name:  Linux
    Kernel Version:  3.6.11-gentoo
    X Server vendor:  The X.Org Foundation
    X Server release:  11301000

Video Card:
    Driver:  NVIDIA Corporation GeForce GTX 460/PCIe/SSE2

    Driver Version:  4.3.0 NVIDIA 313.18
    Desktop Color Depth: 24 bits per pixel
    Monitor Refresh Rate: 60 Hz
    VendorID:  0x10de
    DeviceID:  0xe22
    Number of Monitors:  2
    Number of Video Cards Not Detected
    Primary Display Resolution:  1920 x 1080
    Desktop Resolution: 3840 x 1080
    Primary Display Size: 20.04" x 11.26"  (22.95" diag)
                                            50.9cm x 28.6cm  (58.3cm diag)
    Primary Bus: PCI Express 16x
    Primary VRAM: 1024 MB
    Supported MSAA Modes:  2x 4x 8x 16x 

Sound card:
    Audio device: Loopback Mixer

Memory:
    RAM:  16035 Mb

Miscellaneous:
    UI Language:  English
    LANG:  en_US.utf8
    Microphone:  Not set
    Total Hard Disk Space Available:  905512 Mb
    Largest Free Hard Disk Block:  45661 Mb

Installed software:

Recent Failure Reports:
    Wed Jan 30 16:45:50 2013 GMT: file ''/tmp/dumps/assert_20130130114548_1.dmp'', upload yes: ''CrashID=bp-51e40c97-40a0-4f10-951a-936612130130''
    Thu Jan 31 21:13:45 2013 GMT: file ''/tmp/dumps/crash_20130131161344_1.dmp'', upload yes: ''CrashID=bp-4bad6f01-db95-41b0-b6a6-c2aa32130131''
    Sun Feb  3 02:18:08 2013 GMT: file ''/tmp/dumps/crash_20130202211806_1.dmp'', upload yes: ''CrashID=bp-5606dcf9-5da7-4d38-a692-60d352130202''
    Tue Feb  5 21:57:05 2013 GMT: file ''/tmp/dumps/assert_20130205165703_1.dmp'', upload yes: ''CrashID=bp-8c6890bc-12de-42c2-be3e-d848c2130205''
    Fri Feb  8 01:54:58 2013 GMT: file ''/tmp/dumps/assert_20130207205457_1.dmp'', upload yes: ''CrashID=bp-663f1f44-a563-4138-830c-9a25b2130207''
    Fri Feb  8 01:59:41 2013 GMT: file ''/tmp/dumps/assert_20130207205940_1.dmp'', upload yes: ''CrashID=bp-1df6ec84-452f-4ab9-b62f-2ab742130207''
    Fri Feb  8 02:48:08 2013 GMT: file ''/tmp/dumps/crash_20130207214807_1.dmp'', upload yes: ''CrashID=bp-0364a5ad-d2f8-42cf-b9f0-f905b2130207''
    Fri Feb  8 03:37:31 2013 GMT: file ''/tmp/dumps/assert_20130207223730_1.dmp'', upload yes: ''CrashID=bp-927e9f65-dbcb-4b66-a371-8155d2130207''
    Mon Feb 11 06:02:22 2013 GMT: file ''/tmp/dumps/assert_20130211010220_1.dmp'', upload yes: ''CrashID=bp-b88d4a55-4cc2-4767-8e23-18cdb2130210''
    Mon Feb 11 22:26:59 2013 GMT: file ''/tmp/dumps/crash_20130211172658_1.dmp'', upload yes: ''CrashID=bp-e3ba47d9-0b38-4211-b4a7-c63a62130211''
    Mon Feb 11 22:30:46 2013 GMT: file ''/tmp/dumps/crash_20130211173044_1.dmp'', upload yes: ''CrashID=bp-c8b7d205-c5ea-42d3-a534-c09282130211''
    Mon Feb 11 23:11:46 2013 GMT: file ''/tmp/dumps/crash_20130211181145_1.dmp'', upload yes: ''CrashID=bp-a43ea87b-dd3a-4489-a831-c8e812130211''

The last three crash dumps from Feb. 11 are relevent, the others are not.

This is a low-priority issue due to the limited exposure to this scenario.