How to attach media item created during migration to media field in node
When I have done a similar type of migration at that time I did the following steps:


First I created media type Audio in Drupal 8 and it's fields.
Then I exported all audio file details and FID from D7 and created csv file.
Written migration script to import audio into media.


Example: 

source:
  plugin: csv
  path: 'csv file path'
  header_row_count: 1
  keys:
    - fid
  type: audio
  constants:
    bundle: audio
process:
  mid: fid
  bundle: constants/bundle
  langcode:
    plugin: default_value
    source: language
    default_value: und
  name: filename
  uid: uid
  created: upload_date
  changed: upload_date
  field_media_audio_file/target_id: fid
destination:
  plugin: 'entity:media'
  migration_dependencies: {  }
migration_dependencies: null



The same csv I used to import data into file entity and written migration script.


Example:

source:
  plugin: csv
  path: 'csv file path'
  header_row_count: 1
  keys:
    - fid
process:
  uid: uid
  fid: fid
  filename: filename
  created: upload_date
  changed: upload_date
  filemime: mime
  uri: uri
destination:
  plugin: 'entity:file'
  source_path_property: filepath
  destination_path_property: uri
migration_dependencies: {  }



After that, I attached the FID to my node field.



process:
  field_audio/target_id: fid

  
  I am keeping media id, FID of Drupal 8 same as FID of Drupal 7. So we
  can attach the same FID to the node directly.