Are the commas needed?

Here the comma helps separating the right part from the left part because you introduce a form of specialization of the last statement. It is equivalent to adding « that is » or « which is »: today I listened to a song which is Penny Lane. On the other hand « a song Penny Lane » would refer to a style or type of song rather than one song in particular. If you wrote today I listened to THE song Penny Lane, you would not need a comma.