Privately held International Image Services Inc., a leading Santa Monica-based firm that manages and digitizes film footage for the entertainment industry, said it has agreed to be bought for $24 million in cash and stock by a Midwestern software maker. Sonic Foundry Inc., based in Madison, Wis., makes computer tools that let companies edit audio and video files, and later stream the data over the Internet. The two companies signed a letter of intent Friday, and the deal is expected to close in a couple of months, pending shareholder approval. Publicly traded Sonic Foundry, which has a staff of 300, said it will pay for the deal by issuing 600,000 shares of stock--priced at $20 a share, as well as spend $4 million to cover International Image's outstanding debt and an additional $8 million in cash. International Image, which also has offices in Toronto, has about 100 employees and pulled in nearly $10 million in revenue last year, officials said. Sonic reported a net loss of $2.5 million for its fiscal second quarter ended March 3. No layoffs are expected, according to officials at both companies. Sonic Foundry's shares rose $2.75 to close at $12.50 on Nasdaq.