There's a flag named "use_pulseaudio" in the Chromium tree in qtwebengine module which apparently switches Pulseaudio support in Chromium (it's used and set e.g. in qtwebengine/src/3rdparty/chromium/media/media.gyp). Qt configure script has also autodetection part and a couple of keys to detect and/or switch on/off Pulseaudio support.
The problem is that the result of Pulseaudio support state of Qt configure doesn't affect at all "use_pulseaudio" flag. Actually it should be "use_pulseaudio=0" when PA was disabled during config and "use_pulseaudio=1" when it was enabled. The side effect is that Qt 5.4.0 fails to build Chromium on systems without Pulseaudio.