After 1 google search, here is a link that gets exactly what you want. Hope that helps!
Direct quote from the link :
And then I found Syncness. It doesn't require a PC client as it connects straight to network shares over your WiFi. Requires minimal setup (see below), and then you can have a directory of music on your PC automatically syncing to your phone every night (or just manually whenever you want). It costs about £1, but is well worth it.
Put some music in a dir on your PC, and share it: R click > Sharing Options (then it might ask you for permissions to install some sharing libs). Now open Syncness on your phone and create a new profile and give it a name e.g. Music. Click the "Share" folder icon, and let it scan your network for shares, and then select your share from the list. Next click the "Local" folder icon and select your Music dir (if you don't already have one, you may need to connect your phone to your PC and create it first). Set "Sync Type" to "Push To / Pull From Remote", check "Active" (so it appears in your "Active profiles" list), and also check "Allow file deletions", so it can wipe old music you don't want any more. Then enter your Ubuntu name/pass (I left Domain blank), and do Menu > Save. Then you can setup a sync schedule if you want, but I don't change my music that often, so I just do it manually by pressing the big blue SYNCHRONIZE button on the home-screen.