Because the Federal Communications Commission might require American Telephone & Telegraph to refund $100 million, auditors for AT&T gave "qualified opinions in the company's 1978 and 1979 financial statements.

Coopers & Lybrand, At&T's accountants, said the pending FCC inquiry into the company's 1978 earnings caused it to reverse its unqualified opinion of AT&T's 1978 earnings and to qualify its opinion on 1979 profits as well.

At issue in the FCC inquiry is whether AT&T exceeded the approved rate of return it was permitted to earn on its interstate phone call revenues. In 1976, the FCC said AT&T could earn between 9.5 percent and 10 percent. In 1978, the company earned about 10.2 percent.

When an accounting firm gives a "qualified" opinion, it means that the auditor cannot certify for sure that the financial statements are accurate.

In this case, Coopers & Lybrand is saying that AT&T's 1978 earnings might be $100 million less than AT&T states, because of the inquiry.

The giant telephone utility had about $5.12 billion in earnings in 1978 on revenues of $40.99 billion. Last year the company earned $5.52 billion on revenues of $45.41 billion.

Coopers & Lybrand said, "The eventual outcome of this (FCC) inquiry and the effects, is any, on these financial statements are uncertain."

W. Warren Brown, assistant comptroller of AT&T, said the qualified statement is of no concern to the phone company. The qualification merely reflects a "minor cloud" over the earnings statements he said.

Even if AT&T wer required to make the full $100 million in refunds that some consumer groups claim is necessary, it would reduce earnings by less than 2 percent.

AT&T said that it did not make any excess profits in 1978 and this week told the FCC -- in a separate rate filing request -- that it needs to make 13 percent on its interstate investments, compared with the 9.3 percent it will make in 1980 at current rates.

The company asked for 10.5 percent, two-step rate increase this year, its first general increase since 1976. The increase, which would take place in June and October, would add about $1.2 billion to revenues and $609 million to net income.

AT&T told the FCC that it will need to raise more than $6 billion this year compared with the $5.2 billion it raised in stocks and long-term debt offerings in 1979. With interest rates at the current record levels, the company needs a higher rate of return on its investments, officials aruge.

Today, for example, the Southern New England Telephone Co., a member of AT&T's Bell System, paid 14.29 percent on the $150 million of 40-year debentures it sold to the public. Last week, another Bell System phone company -- Pacific Telehphone & Telegraph Co. -- paid 15.5 percent on $100 million of 40-year bonds. Pacific, which has a lower bond rating, had to pay a higher interest rate than New England Telephone.

But in both cases, the rates the two phone companies paid were by far the highest either has had to pay to raise money.