Couple questions first How do you edit a song so all you hear is the music? and secondly how do you loop a part of a song?
Welcome to the Magix Sound Forge users community @Sandy-Nared.
Which version of Sound Forge do you have?
Read the 'Sticky' at the head for the main Sound Forge forum page.
Are you stating the song has elements you want remove? (vocals for instance)
Has the song been mixed or are the elements on separate tracks?
To loop a song, other sounds (or part of), select the region you wish to loop. Choose: "Loop playback" in the 'Options' menu (or use the keyboard shortcut: Q). Commence playback and the loop region will repeat until you select Stop or Pause.
Select the menu command (or shortcut) again to remove the loop action.
I have Audio Studio 14. I want just the music of a song, for example I like tv theme songs, and some you love for just the music like Threes Company, The Flintstones and The John Davidson Show theme which I downloaded from youtube to mp3 and mp4, and you get like announcements over the theme, so I want to fade out the announcement and keep the theme only
You might be able to remove vocals from audio with https://ultimatevocalremover.com/
I'm not sure how well it will work on voiceovers because the system was trained to remove singing but it might be worth a try. This will probably not work to remove sound effects.
As @SP. stated "You might be able to remove vocals" ('might' being the keyword), Vocal removers do not always work, even for singing vocals which they are primarily designed for. It depends on how it was mixed. You may be able find the theme songs online without the announcements. I recall cassette tapes were available that had most or the 50-60s TV themes.
iZotope's RX Advanced and Steinberg's SpectraLayers Pro will likely do better job which uses AI to separate the components. But they ain't perfect either and both are costly.. though SLP is included with the Sound Forge Pro 16 Suite version and IMO is worth the upgrade cost alone if serious restoration work is needed. There is lower cost software that can separate the components too, but I have not demo'd any of them and would not expect much.