Loose folders full of tracks from mixed artists and/or albums are not supported.
Plex will always try to ingest them into its database, organized by
Music > Artist > Album > Tracks
(Plex may even fail to properly recognize the music if it is not organized on disk by the above structure. So I recommend you to stick to it.)
So, all the music is ingested first into the database with a fixed “OCD-like” organisation. 
If you then want a way to select some of these tracks to be played, use playlists.
Or use the new Radios, which are some kind of automatic playlist generators which make a playlist based on certain conditions.
You can also make ‘smart playlists’ which use several conditions you define to dynamically generate a playlist. [HowTo] Create Playlists
