* American Express yesterday indicated it was interested in an offer by a Time Inc. subsidiary and a Denver-based cable company to buy Warner Amex Cable Communications Inc. for $750 million in cash.

American Television & Communications Inc., the Time subsidiary, and Tele-Communications Inc. also offered to assume Warner Amex's debt, which was about $550 million at the end of 1984.

Since December 1979, Warner Amex has been operated as a joint venture of American Express Co. and Warner Communications Inc.

Gayla Sangallo, a spokeswoman for American Express, read a statement from the company saying that "American Express is favorably disposed toward the offer made by ATV and TCI and is working to resolve any open issues surrounding the proposal." She declined to elaborate on what the open issues were.

Stewart Blair, vice president of Tele-Communications, said executives of American Express had "responded favorably" to the offer, but that Warner had not yet done so. He said the buyers had requested a decision on the offer "within a matter of days."

Jeffrey Holmes, a Warner spokesman, said the company "had received the offer and we are studying it." He said he did not know when a decision might be made.

Under Warner Amex's joint venture agreement, a partner who wants to sell out must offer the other partner its shares in Warner Amex before disposing of them.

Warner Amex has 1.2 million subscribers and owns interests in MTV Networks Inc. and Showtime-The Movie Channel, two providers of cable programming. American Television has about 2.5 million cable subscribers; Tele-Communications has 3.7 million subscribers.

Warner Amex has trimmed its debt in the past year, cutting it to $550 million at the end of 1984 from $852 million at the end of 1983. It lost $25 million in 1984, an improvement over the previous year, when it lost $99 million.

* Mutual fund sales hit a record $9.5 billion in April, up from $6.3 billion the previous month and $4.4 billion a year earlier, according to the Investment Company Institute.

For the first four months of this year, mutual fund sales totaled $29.5 billion, compared with $16.3 billion for the same period in 1984. Sales of income funds, particularly government income funds, topped $4 billion, almost double March volume.

Alfred P. Johnson, ICI's chief economist, attributed the boom to decreasing short-term interest rates and favorable trends in the stock and bond markets, as well as heavy IRA investments. add f Two subsidiaries of International Bank, a Washington-based "merchant bank," have decided to sell their 10 percent interest in Foster Wheeler Corp., a producer of electrical generating plants.

The 3,194,920 shares of Foster Wheeler are owned by International General Industries, Inc., a packaging company, and by Financial Security Group, the International Bank's property and casualty insurance division. At $13.13, yesterday's closing price, the sale would be worth $41.9 million.