The woman is playing a viola da gamba; the man with his back to the viewer, a lute; the man on the right, a flute; and the standing man, a violin. A figure of a boy in the background was painted out when the painting was cleaned in 1969-70, because it was then considered of inferior quality. It is, however, probably by Olis himself. http://www.nationalgallery.org.uk/paintings/jan-olis-a-musical-party
Music in the Baroque: Companion Website by Wendy Heller 