The Sansa will be quite happy mounted in Ubuntu as an MSC volume (settings > system settings > USB mode > MSC).
Your audio files can be Ogg Vorbis, FLAC, MP3, or WMA. To navigate to your files, the metadata is needed in the form of ID3v2.3 ISO 8859-1 (latin-I) tags. I believe EasyTag is most popular with the linux platform.
If your music files have been transferred, and do not have tags embedded, try looking under “unknown” and see if they show up.
Another option for starters is Folder View. Scroll down under Music > Folders and navigate to them this way.
The Sansa can choke on formats like WMA Lossless or WMA Pro, as these aren’t supported.
Let us know the format of your audio files, and we can work the details out.
The Sansa has a group of sample audio files, loaded in MTP mode, that will give you a feel for searching via the ID3 tags (Album / Genre / Artist / Song). See that these files will play. In Folder View, they will be under Folders > Internal Memory > MTP > Music.
We have plenty of linux fans here (as soon as they venture away from the Christmas tree this afternoon!) My brother infected my brain with the awesome interface of the KDE desktop with Knoppix a few years ago; I run this via CD. I am going to build a proper KDE environment, looking at Kubuntu. Jambo!