I just did a google search for this.
It seems we are not alone in trying to run the windows 7 sound recorder from another application.
Check out http://python.6.n6.nabble.com/Issuing-a-command-via-subprocess-other-methods-in-Windows-7-td4494012.html
The one critical piece of information you did not supply here is
that you are running 64-bit Windows 7. That's the root of the problem.
SoundRecorder.exe is a 64-bit executable, and lives in
\Windows\System32. When you are a 32-bit process on 64-bit Windows, and
you try to refer to files in \Windows\System32, the operating system
"helpfully" rewrites the path so you actually refer to
\Windows\SysWOW64, which is where the 32-bit executables and DLLs live.
Unlike almost every other utility, there is no 32-bit version of
SoundRecorder.exe, so subprocess fails to find the executable and dies.
Cmd.exe is a 64-bit process, so it looks in the real System32 and finds it.
It would seem then your only solution would be to find a 3rd party sound recorder.