How to convert a ‘.pages’ file to something unix-y

OpenOffice comes with the unoconv program to perform format conversions on the command line.

unoconv -f csv filename.xlsx

For more complex requirements, you can parse XLSX files with Spreadsheet::XLSX in Perl or openpyxl in Python. For example, here's a quickie script to print out a worksheet as a semicolon-separated CSV file (warning: untested, typed directly in the browser):

perl -MSpreadsheet::XLSX -e '
    $\ = "\n"; $, = ";";
    my $workbook = Spreadsheet::XLSX->new()->parse($ARGV[0]);
    my $worksheet = ($workbook->worksheets())[0];
    my ($row_min, $row_max) = $worksheet->row_range();
    my ($col_min, $col_max) = $worksheet->col_range();
    for my $row ($row_min..$row_max) {
        print map {$worksheet->get_cell($row,$_)->value()} ($col_min..$col_max);
    }
' filename.xlsx >filename.csv

In response to your update: depending on how you're downloading the video, you may not be getting the best quality version that YouTube is hosting. If you're using clive (or cclive), -f best will get the best it can.

If you want to reliably not lose any data when pulling audio out of a video file, convert it to .wav or ensure you're not re-encoding.