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Google Audio may be headed for an unexpected roadblock, according to an analyst's report: Though a critical mass of advertisers seems interested in pursuing Google's radio advertising, it may not have access to enough radio time for advertisers to sufficiently test the initiative.

Several news reports, however, have said that Google is heavy into negotiations with CBS Radio to buy large amounts of air time, writes Market Watch (via MediaBuyerPlanner). If talks are successful, Google could avoid airtime shortage, says RBC Capital Markets analyst Jordan Rohan. If not, the rollout of Google Audio could be delayed until Google can strike a deal with CBS or another radio giant.

Apparently, Google radio ad sales execs have changed their approach during sales calls, Rohan pointed out in his note, issued on Monday. Rather than strictly selling the efficiency and efficacy of Google Audio, they are now pushing a combination of radio and internet ads.

A Google spokesman said in an email that advertisers have access to more than 700 stations spanning "more than 200 metro areas," and that more stations and inventory will be available to an increasing number of advertisers next year.