Voice: Your brand personality described in an adjective. For instance, brands can be lively, positive, cynical, or professional.
Tone: A subset of your brand’s voice. Tone adds specific flavor to your voice based on factors like audience, situation, and channel.
Essentially, there is one voice for your brand and many tones that refine that voice.
Voice is a mission statement. Tone is the application of that mission.
Another way of looking at voice is through a four-part formula suggested by Stephanie Schwab, writing for Social Media Explorer. She takes the general topic of voice and breaks it down into not only tone but also character, language, and purpose. In the graphic below, she uses adjectives to define each different area of the overall brand voice: