Bug 443329

SUMMARY
Since version 21.08.1, JuK always starts muted. I've dug a bit into the source code (not faimiliar with it, nor with the language, programming in general, or frameworks), and I think the problem comes from getting into a loop as follows:

- File 'playermanager.cpp', method 'PlayerManager::setupAudio()', sets a listener 'PlayerManager::setVolume' for 'AudioOutput::volumeChanged'.
- File 'playermanager.cpp', method 'PlayerManager::setVolume', has a line 'emit volumeChanged(volume)' right after setting the volume, which causes it to call itself again.
- After a while from getting stuck in this loop, somehow the volume gets set to NAN or -NAN (passing that as a parameter to 'PlayerManager::setVolume'), at which point it sets the volume to zero and stops changing it thereafter as it compares to the same volume that was set.

If I remove that line, the issue of starting muted seems to be fixed, but every time it starts, it decreases the volume by 1, which I guess happens because of two issues:
- It convers fractional volumes 0-1 between 'double' and 'float', which loses precision. For example, it turns a number like 0.5 into 0.499something.
- It then uses 'int(100 * float)' and 'int(int * 0.01)' for rounding volumes to integers/fractions, when it should be using 'std::round' instead, which combined with the issue above makes it pick a smaller volume in an integer 0-100 scale.

From some experiments of my own changing those seems to fix the issues, but I don't know if it introduces any new issues along. In any case, here is a callback trace of the moment in which 'PlayerManager::setVolume' is called with a NAN value:
PlayerManager::setVolume(PlayerManager * const this, qreal volume) (/home/david/del/juk/juk-21.08.0/playermanager.cpp:437)
QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<double>, void, void (PlayerManager::*)(double)>::call(void (PlayerManager::*)(PlayerManager * const, double) f, PlayerManager * o, void ** arg) (/usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:152)
QtPrivate::FunctionPointer<void (PlayerManager::*)(double)>::call<QtPrivate::List<double>, void>(QtPrivate::FunctionPointer<void (PlayerManager::*)(double)>::Function f, PlayerManager * o, void ** arg) (/usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:185)
QtPrivate::QSlotObject<void (PlayerManager::*)(double), QtPrivate::List<double>, void>::impl(int which, QtPrivate::QSlotObjectBase * this_, QObject * r, void ** a, bool * ret) (/usr/include/x86_64-linux-gnu/qt5/QtCore/qobjectdefs_impl.h:418)
libQt5Core.so.5![Unknown/Just-In-Time compiled code] (Unknown Source:0)
libphonon4qt5.so.4!Phonon::AudioOutput::volumeChanged(double) (Unknown Source:0)
libQt5Core.so.5![Unknown/Just-In-Time compiled code] (Unknown Source:0)
libQt5Core.so.5!QObject::event(QEvent*) (Unknown Source:0)
libQt5Widgets.so.5!QApplicationPrivate::notify_helper(QObject*, QEvent*) (Unknown Source:0)
libQt5Core.so.5!QCoreApplication::notifyInternal2(QObject*, QEvent*) (Unknown Source:0)
libQt5Core.so.5!QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) (Unknown Source:0)
libQt5Core.so.5![Unknown/Just-In-Time compiled code] (Unknown Source:0)
libglib-2.0.so.0!g_main_context_dispatch (Unknown Source:0)
libglib-2.0.so.0![Unknown/Just-In-Time compiled code] (Unknown Source:0)
libglib-2.0.so.0!g_main_context_iteration (Unknown Source:0)
libQt5Core.so.5!QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) (Unknown Source:0)
libQt5Core.so.5!QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) (Unknown Source:0)
libQt5Core.so.5!QCoreApplication::exec() (Unknown Source:0)
main(int argc, char ** argv) (/home/david/del/juk/juk-21.08.0/main.cpp:122)

(source code was taken from debian sid source package)

STEPS TO REPRODUCE
1. Start JuK.

OBSERVED RESULT
Starts muted, and after that issue is fixed, stars with a lower volume every time.

EXPECTED RESULT
Should start with the same volume that's stored in the configuration file.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.21.5
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION