Swami Sivananda is singing a Shanti Mantra. You can find the text to all the Shanti Mantras here.
After the mantra Swami Sivananda is talking about the Katha Upanishad.
The Katha Upanishad (Sanskrit: कठोपनिषद् or कठ उपनिषद्) (Kaṭhopaniṣad) is one of the mukhya (primary) Upanishads, embedded in the last short eight sections of the Kaṭha school of the Krishna Yajurveda. It is also known as Kāṭhaka Upanishad, and is listed as number 3 in the Muktika canon of 108 Upanishads.
In all the Yoga Vidya ashrams we offer several English seminars every year. You can find all of them by clicking here.