Financially ailing Baldwin-United Corp. has "voluntarily" suspended sales of its once-high-flying single premium deferred annuities until state insurance regulators can decide whether Baldwin insurance subsidiaries are healthy, the company has announced.

In addition, University Life Insurance Co.--one of the two companies that has sold billions of dollars of the annuities since 1979--now is operating under the "overall supervision" of the Indiana insurance department, Baldwin said.

Arkansas-based National Investors Life Insurance Co. was the other chief seller of the annuities, which former Arkansas insurance commissioner W.H.L. Woodyard called probably the fastest selling insurance product since life insurance companies began to be regulated in the 19th century.

Both the Arkansas and the Indiana insurance departments are concerned about the health of the Baldwin-United insurance subsidiaries, because the insurance companies have a large portion of their investments in stocks of companies that are part of the Baldwin empire. They are concerned that those stocks may be overvalued because of Baldwin's financial problems--among other things, the parent company owes about $900 million to its banks that it cannot repay right now.

Baldwin--which last year sold $1.6 billion of the annuities and nearly that much in 1981--said it suspended further sales until the insurance regulators resolve issues surrounding those so-called affiliated securities. Those premiums accounted for nearly 45 percent of Baldwin's $3.6 billion in income in 1982.

By itself, however, the suspension means little to Baldwin's cash flow. Sales began to slow early in the year when news of Baldwin's problems became public, and investors cashed in $76 million of those annuities during the three weeks ended April 8, even though they had to take a 5 percent penalty. During the same period, sales totaled only $27 million.

Ron Taylor, assistant Arkansas insurance commissioner, said last week that, based on their Dec. 31 financial reports, he believes National Investors Life and its sister company, National Investors Pension Insurance Co., are solvent, based on their Dec. 31 financial reports. The two companies are scheduled to file updated reports this week.