When dementia patients can play music and sing together, is that because angelics or human light beings are using them as instruments to give them the experience as a blessing?

This can be taking place. It is also the province of higher self to provide encouragement and upliftment for the human extension in the physical. All come from the divine realm and belong within the divine realm. But music, in general, is a divine human partnership when done freely and with feeling and passion. In fact, music calls forth the Divine and it will be onboard flowing through you as you let go and engage in song or through an instrument to bring forth a joyful noise. This is Creator’s plan and the reason why humans respond to music as well as beauty finding its way through artistic expression as a delight to the senses. It is particularly on display when an otherwise impaired individual suddenly seems to come to life in unexpected ways and demonstrates proficiency beyond their seeming capability. While individuals with impairment can have an intact consciousness, but an inability to express it through the body, the fact this magic happens is an overriding of the limitation through divine intervention, as divine energy flows into the person and bridges the gap and allows a renewed control of the body in some way to allow producing music or song. It is a divine collaboration in all respects.