Seoul says it plans to install many more loudspeakers at the border
Late on Monday it began playing radio programmes, soon to be broadcast via border loudspeakers.
Seoul has already suspended trade ties with Pyongyang over the sinking.
China's envoy on North Korea is in Seoul for talks, amid pressure on Beijing to back a response to the incident.
The Cheonan sank near the inter-Korean maritime border on 26 March.
An international panel says a torpedo fired from a North Korean submarine sent the ship down - but Pyongyang denies this.
South Korea's defence ministry said the first radio programme, entitled Voice of Freedom, went out on Monday evening. Broadcasts would take place three times a day, a spokesman said.
He said the programme would be broadcast through high-performance loudspeakers that will be installed along the demilitarised zone.
"Initially we are installing loudspeakers at 14 places along the DMZ (Demilitarised Zone). The installation requires several months of work," the spokesman told AFP news agency.
South Korea says it will also drop propaganda leaflets into the North to tell people about the Cheonan incident as soon as possible, and set up giant electronic billboards to flash messages.
It plans to refer North Korea to the UN Security Council, and is seeking a unified international response to the incident.
The US has backed South Korea, condemning the incident and confirming late on Monday that it will hold joint naval exercises with the country.
But China's attitude is seen as key, because it holds a veto in the Security Council and has in the past been reluctant to impose tough measures on Pyongyang.
It has so far called for calm and restraint.
On Tuesday Wu Dawei, China's special representative for Korean affairs, arrived in Seoul for talks with Foreign Minister Yu Myung-hwan.
US Secretary of State Hillary Clinton is also due in Seoul for talks on Wednesday.
On Monday she described the situation on the Korean peninsula as "highly precarious", and said the US was working to avoid an escalation in the crisis.
Source: BBC News.
Clearly they should be pumping pop music into the DMZ. Nothing to make you realize there's a better deal out there than autotune, pop-rappers, and the distilled sound of kilos of sparkles. ;)