A few recommendations...
Some CubeOwners seem to like the Creative Labs Sound Blaster MP3 + USB sound system.
Connections and Controls: Line In (Gold-plated, RCA pair), Mic In (Gold-plated, 1/8"), Optical S/PDIF In, Line Out (Gold-plated, RCA pair), Line/Headphone Out (Gold-plated, 1/8"), Optical S/PDIF Out, Analog headphone volume control, Analog/Digital output selector. More in the Forums
The Griffin Technology iMic USB Audio Interface: The iMic™ universal audio adapter is a USB device that adds a stereo input and output to your Cube. This allows the connection of virtually any microphone or sound input device. The iMic supports both line and Mic level input as well as line level output for any USB capable computer. Although in principle the iMic can record and emit sounds at 24-bit resolution, Apple's audio manager limits it to just 16-bit resolution.
Installing the iMic is trivial - plug it into a USB port and Apple's built-in audio driver takes over. Once your Mac has recognized the iMic, you'll see new audio input and output sources in the Sound control panel. Select a source, and you can immediately record sounds from anything plugged into the microphone port or listen to headphones or speakers plugged into the output port (depending upon which port you've selected).
For casual recording, the iMic is recommended; but for professional use, you may want to take a look at the Griffin Powerwave USB Audio Interface & Amplifier, which has stereo RCA jacks and minijack ports and includes the cables necessary to connect the unit to your stereo or iPod.
If MIDI is your thing, you can use a USB or FireWire MIDI Interface.
Luckily, USB Audio products such as the ones made by Edirol will allow you to take both analog signals via an RCA plug (from a CD player or a guitar, for example) and digital signals (from a DAT recorder, for example) from either S/PDIF (Sony/Philips Digital Interface Format) or optical connections, and pump it all into your G4 Cube over USB. Because of USB's relatively slow transfer speed, however, (12 Mbps max) this limitation doesn't make the Cube an optimal choice for audio professionals. They'd still be better off with a G4 or G5 desktop model that has PCI slots for installing a digital sound card or a PowerBook with PCMCIA card.
The M-Audio Firewire Audiophile is a compact, FireWire-compatible audio/MIDI interface that takes the legacy of M-Audio’s award-winning Audiophile line to the next level. 4 x 6 audio I/O combines with powerful on-board mixing in a mobile package that’s perfect for live performance, home recording, and digital DJ work. Stereo headphone output with A/B switching between assignable sources allows precuing for DJs and mobile musicians—and the assignable aux bus is perfect for creating dedicated headphone mixes and effect sends. You also get zero-latency hardware direct monitoring, ultra-low latency ASIO software direct monitoring, and 1 x 1 MIDI I/O. FireWire Audiophile even has AC-3 and DTS passthrough for surround sound playback. Cube owners be warned, the M-Audio requires a G4 500MHz w/ OS 10.1.5 or OS 10.2.6 or greater and carries an MSRP of $350.
