Fix EXT output in format switching

• Related issues: #1566
• Requested reviewers: @sagnowski or @pihlajakuja

Reason why this change is needed

• The format switching with EXT output crashes.

Description of the change

• Fix format switching with EXT output:
  • set correct number of output audio channels
  • open metadata output files

Affected operating points

• All is BE except of format switching and EXT output.

Related to #1566