Sirius XM hit a 12-year low this week. It's time to pump up the volume.The major market indices may have hit fresh all-time highs this week, but Sirius XM Holdings (SIRI 3.96%) investors weren't invited to the party: Shares of the satellite radio provider hit a 12-year low this week.There's no shortage of explanations for the former battleground stock losing the war on Wall Street. Growth has slowed to a crawl. Its subscriber base may have also plateaued last year.Making matters worse, the conversion of the ...