There’s nothing baked into Ghost. However, if you’re comfortable editing the theme, you could edit post.hbs to include something like this (note - needs to be within the existing {{#post}}..{{/post}}:
{{#get "posts" limit=1 filter="slug:{{slug}}-audio"}}
{{#foreach posts}}
{{#if access}}
{{content}}
{{else}}
You need to be a member of tier 1 to use the audio
{{/if}}
{{/foreach}}
{{/get}}
So then you’d make your regular post (minus audio) with the slug my-post-slug and access set to everyone who can have access (tiers 1&2), and then make an additional post that just contains the audio narration, with access set to tier 2 only.