Did you download Qt's sources ?

Indeed, you can't do it without modifying Qt even if you add a control to the plugin to modify that parameter.

QtGstreamer provides a QML sink so you don't have to implement it.

It looks like you're using the Ubuntu packaged Qt and not the one you installed using the installer.
You should check the Kit in QtCreator you're using.

@JKSH said:

What about the "audio", "mediaservice", and "playlistformats" folders? The "Initial Deployment" told you to copy the contents of the "plugins" folder too.

I did copy everything that's mentioned in the "Initial Deployment" (see below). In my above post I only mentioned what I thought was directly related to QtMultimedia, sorry for the misunderstanding.
http://wiki.qt.io/Deploy_an_Application_on_Windows:

Copy the following into C:\Deployment\

The release version of MyApp.exe All the .dll files from C:\Qt\5.2.1\mingw48_32\bin\ All the folders from C:\Qt\5.2.1\mingw48_32\plugins\ (If you used QML) All the folders from C:\Qt\5.2.1\mingw48_32\qml\

@JKSH said:

What video format is it? Did you install any codecs on your development PC?

It is MP4 (codecs reported by VLC: Video: H264 - MPEG-4 AVC (part 10) (avc1), and Audio: MPEG AAC Audio (mp4a)). I also tried a MOV and a 3GP video, but none of these can be read by my software on my test PC. But all of those videos can be read by Windows Media Player on my test PC, and also by my software on my development PC with the Qt directory renamed.
AFAIK I didn't explicitly install any codec on my development PC. The codecs set on both PCs match exactly (as reported by Windows Media Player), except for a CyberLink audio codec.

@JKSH said:

use Sysinternals ListDLLs to show you the DLLs that it uses

Using ListDLLs, I see on my development PC that 10 DLLs get loaded as I open the video. On my test PC, the DLLs that get loaded as I open the video are the 10 same ones, except for the version build number (e.g. 12.0.9600.16477 instead of 12.0.9600.17415). Could the problem be these versions differences? In this case how can I fix this, because even if I distribute the newest versions with my software, the old ones still get loaded.

Thanks a lot

Hi and welcome to devnet,

Qt Multimedia might not be the best framework for that task. However what you can try is to retrieve audio from both the file using QAudioDecoder and the microphone using QAudioInput and then use a library like DSPFilters to the mixing part before you output the result with QAudioOutput

Hope it helps