This usually happens because kdenlive is not smart enough to deal with multiple types of files, especially different frequency. If you take a 44.1khz audio track and then add a 48khz track, you will definitely get the pops and clicks. Use a tool like audacity or similar to do two things, flatten the audio tracks to mono and at least make sure both tracks have the same frequency. I recommend changing both to 44.1khz. After Idid this and reimported the trac........

Cannot register the hard disk '/media/Debian8.vdi' {d01efabc-aacf-4614-b29c} because a hard disk '/home/VirtualBox VMs/Debian/Debian8.vdi' with UUID {d01efabc-aacf-4614-b29} already exists. This is what happens if you move the VDI to another location. The easiest way is to delete/remove the machine from VBOX and then reimport it from the new location.........