What happens if you write CD from CLI?
https://www.FreeBSD.org/doc/handbook/creating-cds.html
In particular, you will want to see if cdrecord(1) can burn CD before using GUI.
Are you sure you don't have a batch of bad CDs? If that is not the case you need to configure your device. Who configured the device to begin with? IIRC (and I was one of first people to use K3b on FreeBSD many years ago) there was a whole section in FreeBSD handbook how to configure CD device. Only after that you have a chance of getting K3b to work properly. By the time you configure the device, learn manually how to use madplay to convert mp3 to wav file you will realize you don't need K3b anyway.Tried cdrecord and result was same.
I have a problem with sysutils/k3b-kde4 when I'm writing something to CD. The writing process finishes without any errors or problems but CD is empty. Writing to DVD works.
This is odd. The dvd writing backend is sysutils/dvd+rw-tools and it uses sysutils/cdrtools in its tool chain. It has been awhile since I used a gui interface to burn media but IIRC there are configuration options to select the device. I do not know if k3b allows the configuration of different devices for cd vs dvd's. I would also consider starting k3b from the command line and watching for any error messages in the terminal. To add to the confusion, FreeBSD packages both systutils/cdrtools and it's fork sysutils/cdrkit.