The verb to feel is a special verb.  In your example, it's both ergative and copular. 

Some languages have a voice that is different from both the active and the passive voices, called the middle voice.  English doesn't, or at least English grammar doesn't have a separate form or structure to mark the middle voice.  Ergative verbs in English use active voice constructions. 

This farmer grows wheat. 
Wheat is grown in the Great Plains. 
Wildflowers grow in my back yard. 

The first sentence above uses the transitive sense of to grow in an active voice construction.  The subject is a semantic actor, and the direct object is a semantic patient.  The second sentence uses the same sense of the verb to grow in a passive voice construction.  There is no actor or agent required (although a prepositional phrase can supply one), and the subject is the patient. 

The third sentence uses the ergative sense of to grow.  The subject of this sentence isn't simply an actor or a patient.  It's something like actor and patient combined. 

Suzie smelled the roses. 
The roses smelled wonderful. 

In this pair of sentences we see the transitive and ergative senses of the verb to smell.  In the first, we have subject as actor and direct object as theme.  In the second, we have subject as agent and theme combined and a predicate adjective subject complement. 

Most copular verbs allow either a predicate adjective complement or a predicate nominative complement.  These copular ergatives do not allow predicate nominatives. 

The roses were wonderful.  The roses were a delight.
The roses smelled wonderful.  *The roses smelled a delight.

That last sentence doesn't work with the ergative sense of to smell because "a delight" looks like a direct object instead of a subject complement -- which forces to smell into its transitive rather than ergative sense. 

I feel good. 
Hanging out feels good. 

Both of these have the same grammatical structure as "the roses smelled wonderful."  We have subjects that combine the roles of actor or agent and patient or theme, and a predicate adjective subject complement.  These are both copular ergative structures with predicate adjective subject complements.

Despite the identical grammar, they don't carry the same semantics.  In the first, the subject "I" is able to perceive things.  It makes sense to assign a semantic role of perceiver or experiencer to the subject of the first sentence.  An activity like "hanging out" isn't a sensible perceiver, and we simply assume that someone not mentioned in the sentence experiences the good feeling. 

There is no change in voice between "I feel good" and "hanging out feels good".  There is only a change in what semantic roles those subjects can fill.