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It would be under /run/user/s/gvfs on the filesystem.

Another thing you can do is simply drag the file from Nautilus into the file open dialog of the application you want to open it in, or to a terminal. This should give you the full path to open with the classic POSIX methods for apps not using gvfs.

Although, if it is an Audio CD, it is likely not actually mounted. The underlying VFS support in GNOME/Nautilus just scans the disc and presents the tracks as raw wav files in the UI.

First, you need to understand Linux/Unix mount points a bit better. I wrote this article on the topic a while back, so you might consult it for background information. I'm sure there are other tutorials on this topic you might consult, but I don't have any other URLs handy.

Once you've got a better understanding of mount points, you should realize that your disk sizes are a bit awkward. A complete Ubuntu installation takes about 5-8GiB, although it's common to create a root (/) filesystem in the 10-30GiB range to give some space for growth and temporary files, plus a separate /home partition (as thom suggested) for your user files. (/home is not used in a default Ubuntu installation, but IMHO it should be.) A separate swap partition as large as your RAM or a bit larger is also common. If you add up the needs of root (/) and swap, it's almost certain to be well under half the size of your smaller disk. If you mount your 320GB drive at /home, that means you'll have either a ridiculously oversized root (/) or you'll have unused space on the 160GB disk and no convenient place to mount it.

Some people deal with this issue by using some of the storage space as a partition that will be auto-mounted somewhere under /media by Ubuntu. This is awkward, though, since your user files normally reside in /home, so you'll have to decide what goes where, and if you make bad decisions, you'll run out of space and have to spend time juggling files around to deal with it.

A more sophisticated way to handle the problem is to use Logical Volume Management (LVM), which enables you to group together several partitions (physical volumes in LVM-speak) into a volume group, which you can then carve up into logical volumes that can be any size -- even larger than any given physical volume. Using LVM will therefore enable you to set up a /home partition that spans both your disks -- perhaps 400-450GB in size. The trouble with LVM is that Ubuntu's desktop editions provide poor support for it, so you'll need to jump through extra hoops to use it. (FWIW, Fedora provides much better out-of-box LVM support than Ubuntu.)

I recommend you read a bit about LVM (at the link in the previous paragraph) before making any decision. You might decide that it's more hassle than it's worth, and just deal with your awkwardly-sized disks in some other way; or you might decide to take the plunge and set up an LVM for your computer. The point is to make an informed decision.