If you plan to work with surround sound in Soundtrack Pro or Logic Pro, you'll need an audio interface with enough discrete outputs to accommodate the surround format you plan to work with. For example, if you work in 5.1, you'll need at least 6 outputs (for the 5 channels, plus the LFE channel).
The built-in optical hardware output included with some Macs cannot be used for monitoring surround with Logic Pro and Soundtrack Pro, because it requires a surround encoded audio stream, such as that output by a DVD. Neither Logic Pro nor Soundtrack Pro can playback surround encoded audio; they output discrete audio streams that can be encoded later in the authoring process by an application such as Compressor.