word of mouth: phr. 口头说的话，口头相传；理解为gossip spread by spoken communication。
by word of mouth: adv.口头地=orally，理解为by spoken rather than written means。

【例句】
The interest seems to be word of mouth rather than the advertising so far.
比起做广告,对电影的关注看起来还是口口相传的。

It spends relatively little on advertising, mainly relying on word of mouth to drive sales.
它在广告方面的投入不多，主要依靠人们的口碑来拉动销售。