Ubuntu – Can’t mount or unmount virtual discs- gives option for “force unmounting”

Audio CDs are a bit different - there's no actual filesystem there and as such they can't be mounted. Gnome's gvfs subsystem (which stands for "virtual filesystem") provides a file-like view of tracks on the audio CD, but it does it by directly accessing the device, without actually mounting it. So the tracks on the Audio CD look like files, but in fact they're not. They don't have real filenames, file size in bytes or other attributes of normal files.

You can convert those tracks to normal .mp3 or .ogg files using CD-ripping software if you need.

The "mount points" for gvfs (though they're not real mount points) are created in ~/.gvfs folder. There's supposedly also gvfs-mount command which can list/manage the mounts

Googling for "gvfs cdda" will provide you with additional information if you're still curious.

Clicking the "Install Guest Additions" menu item just adds the iso to the virtual machine. You don't need to click it, because, in your case, the VirtualBoxGuestAdditions.iso is already attached to the virtual machine (as per the Storage tab of the virtual machine settings). It's named like that because the guest is supposed to automount the CD and autorun the installer, but I've never seen this behavior on Ubuntu, so the name is misleading in this case.

From the VirtualBox manual, Chapter 4 (Guest Additions), section "Installing the Linux Guest Additions":

Ubuntu

sudo apt-get update
sudo apt-get upgrade
sudo apt-get install dkms

Reboot your guest system in order to activate the updates and then proceed as described above:

• Insert the VBoxGuestAdditions.iso CD file into your Linux guest's virtual CD-ROM drive [Don't do this, if the Storage tab shows it to be already attached to the virtual machine]
• [open a terminal, ] Change to the directory where your CD-ROM drive is mounted, and execute:
• sudo sh ./VBoxLinuxAdditions.run