Input Assumptions:

• Formatted in VTT
• Empty lines separate each subtitle block
• Sequence lines are optional
• Sequence lines ending with -0 indicate the start of a paragraph

Regular expression used in the process:

Number	Regular Expression	Replace
0	/^(.*-0)[^\S\r\n]*$/gm	"\n$1"
1	/^[^\S\r\n]+$/gm	""
2	/\n(?:[^\r\n]*\n)?[^\r\n]*-->[^\r\n]*\n/g	""
3	/^([^\r\n]+)\n(?!\n)/gm	"$1 "
4	/^\s*|\s*$/g	""

Explanation

0. Add new lines before all new paragraphs, indicated by -0 in the sequence line (ignoring trailing whitespace).
1. Remove whitespace characters on seemingly empty lines.
2. Remove VTT subtitle metadata.
3. Replace new lines inside each paragraph with a space.
4. Remove leading and trailing whitespace.