Hi, I'm having exactly the same issue. It doesn't matter what device I select in the Audio settings, it immediately errors when I press 'Start Stream' or 'Preview Stream'.
I don't know if it's relevant, but I'm running Windows 8 (64bit) and OBS has installed itself in the 'Program Files (x86)' folder.
I have run 'dxwebsetup.exe' and updated any necessary DirectX files. Here is one of the log files:
23:05:10: Open Broadcaster Software v0.522b - 32bit (´・ω・`)
23:05:10: -------------------------------
23:05:10: CPU Name: Intel(R) Core(TM) i5-3330 CPU @ 3.00GHz
23:05:10: CPU Speed: 2993MHz
23:05:10: Physical Memory: 1023MB Total, 1023MB Free
23:05:10: stepping id: 9, model 10, family 6, type 0, extmodel 1, extfamily 0, HTT 1, logical cores 4, total cores 4
23:05:10: monitor 1: pos={0, 0}, size={1366, 768}
23:05:10: Windows Version: 6.1 Build 7600
23:05:10: Aero is Enabled
23:05:10: -------------------------------
23:05:10: OBS Modules:
23:05:10: Base Address Module
23:05:10: 00D80000 OBS.exe
23:05:10: 72750000 OBSApi.dll
23:05:10: 72C80000 DShowPlugin.dll
23:05:10: 72C60000 GraphicsCapture.dll
23:05:10: 72C40000 NoiseGate.dll
23:05:10: =====Stream Start: 2013-05-14, 23:05:10===============================================
23:05:10: Multithreaded optimizations: On
23:05:10: Base resolution: 1366x768
23:05:10: Output resolution: 1364x768
23:05:10: ------------------------------------------
23:05:10: Loading up D3D10...
23:05:10: Playback device Default
Warning -- MMDeviceAudioSource::Initialize(0): Could not create IMMDevice = 80070490
Error: Cannot initialize desktop audio sound, more info in the log file.
It's really frustrating as I've used OBS successfully on Windows 7 before without any problems. Any help would be greatly appreciated...