Problem uploading different files as custom fields with front end post form

For starters, the ‘post_category’ argument only accepts category ID’s, so you will have to find that through your database, or through get_category_by_slug:

$cat = get_category_by_slug( 'music' ); 
$cat_id = $cat->term_id;

Secondly, look at your wp_handle_upload() section. You are passing $file[‘file’] in both cases, where ‘file’ is not the name of either of your upload fields. I believe it should be:

$uploaded_music = wp_handle_upload( $file['music_file'], $overrides );
$uploaded_music_art = wp_handle_upload( $file['music_image'], $overrides );