I will do some testing to try and find which servers have this problem and which don't (and hopefully discover a pattern).
I already found the pattern. This issue occurs when the server in question has "sv_use_steam_voice 0" set in its configuration. This setting forces the server to use the old TF2 voice chat codec instead of the new one.
http://steamcommunity.com/app/221410/discussions/1/882966056682357449/ Here's a link to the thread relating to this issue on the other beta forum. There is no fix that the client could perform (unless of course you have access to the server variables), but more data is always good. Probably.
In some, but not all TF2 servers (I have not been able to find a pattern to which servers work and which don't), I am able to hear others speaking, but not transmit my own voice.
The icon shows on screen that I am transmitting, but no one on the server is able to hear. This does not happen on the same machine under Windows.
When on the main menu of TF2, not in any servers, going into Settings>Voice and testing the microphone, I hear my voice echoed back, and the volume metre lights up. When in a server where the voice chat works, the same things happens. However, in a server where the voice chat does not work, the Voice test does not echo back my voice, and the metre does not light up.
I am currently running arch, but I have also tested under Ubuntu 12.04 LTS (32 bit).
Processor Information:
Vendor: AuthenticAMD
Speed: 3612 Mhz
8 logical processors
8 physical processors
HyperThreading: Unsupported
FCMOV: Supported
SSE2: Supported
SSE3: Supported
SSSE3: Supported
SSE4a: Supported
SSE41: Supported
SSE42: Unsupported
Network Information:
Network Speed:
Operating System Version:
"Arch Linux" (64 bit)
Kernel Name: Linux
Kernel Version: 3.6.10-1-ARCH
X Server vendor: The X.Org Foundation
X Server release: 11301000
Video Card:
Driver: NVIDIA Corporation GeForce GTX 670/PCIe/SSE2
Sound card:
Audio device: Realtek ALC889
Memory:
RAM: 7938 Mb
Miscellaneous:
UI Language: English
LANG: en_NZ.UTF-8
Microphone: Not set
Total Hard Disk Space Available: 178488 Mb
Largest Free Hard Disk Block: 78423 Mb
I am using a Logitech G930 headset.