SUMMARY
I use archlinux (a rolling release linux distro),
since a recent update I notice at startup segmentation fault related to kmixctrl (version 4.6) in dmesg :

[   72.047675] kmixctrl[787]: segfault at 0 ip 0000000000000000 sp 00007ffd2a3ff0e8 error 14 in kmixctrl[5628352c9000+2000]
[   72.047682] Code: Bad RIP value.

You can also trigger the bug by opening a console, then type "kmixctrl", you will notice a segmentation fault message, if you use the last release version of KDE plasma, kde frameworks application, and QT5 (5.15 version).

the backtrace :

(gdb) bt 
#0  0x0000000000000000 in ?? ()
#1  0x00007ffff606b2c1 in ?? () from /usr/lib/pulseaudio/libpulsecommon-13.0.so
#2  0x00007ffff606d860 in pa_socket_client_unref () from /usr/lib/pulseaudio/libpulsecommon-13.0.so
#3  0x00007ffff72c5c58 in ?? () from /usr/lib/libpulse.so.0
#4  0x00007ffff72c5d72 in ?? () from /usr/lib/libpulse.so.0
#5  0x00007ffff7f2b41d in ?? () from /usr/lib/libkmixcore.so.5
#6  0x00007ffff7f2b439 in ?? () from /usr/lib/libkmixcore.so.5
#7  0x00007ffff7f07a15 in Mixer::~Mixer() () from /usr/lib/libkmixcore.so.5
#8  0x00007ffff7f07a2a in Mixer::~Mixer() () from /usr/lib/libkmixcore.so.5
#9  0x00007ffff7f012e7 in MixerToolBox::deinitMixer() () from /usr/lib/libkmixcore.so.5
#10 0x0000555555556796 in ?? ()
#11 0x00007ffff74dc152 in __libc_start_main () from /usr/lib/libc.so.6
#12 0x000055555555683e in ?? ()


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Archlinux 64 bits, 
(available in About System)
KDE Plasma Version:  5.19.4-2
KDE Frameworks Version: 5.73
Qt Version: 5.15

ADDITIONAL INFORMATION
Despite the segfault kmix seems to work (volume applet is Ok).