Symptom #1: Digital Audio File Sounds Tinny or Thin
"I imported an audio file, but it sounds very tinny and thin when I play it back in FCP."
Digital audio is created by sampling analog audio. The digitizer, be it your DV camera, a DAT recorder, or an audio PCI card, analyzes the level and frequency of the analog audio it has been fed. It looks at each wave and generates binary values to describe them. The number of times per second the audio is measured is referred to as its sample rate. The number of binary values it uses to describe each sample is the bit depth. The more frequently the audio is sampled, the higher the quality and the higher the subsequent data rate. Similarly, the higher the bit depth of the sample is, the more accurately the data describes the audio wave.
The minimum sample rate to deliver professional quality audio is usually considered 48 kHz; the minimum bit depth for two-channel stereo audio is 16-bit. As technology improves, 24-bit, 96 kHz digitized audio will become more widely available. We've come a long way from simple audiocassettes, but we still have a long way to go.
The sample rate of 44.1 kHz is quite close in quality to 48 kHz and shares its native bit depth of 16-bit. It is the sample rate of the audio CD standard, and quite good for most serious audio applications.
When your audio is digitized with a low sample rate and bit-depth setting, the resulting audio can sound vastly different from the original. Low sample rates throw out important high-frequency information as they digitize. Low bit-depth settings will reduce the dynamic range of the source audio, and can produce digital artifacts in the new file. You will often end up with a tinny-sounding file similar to a "telephone voice" when sample rates and bit depths are set too low.
Resample the audio. Although you can resample your audio using QuickTime Pro to bring it up to the higher quality sample rate and bit depth of 16-bit and 48K, you will need to start again with the original audio source, since digital audio that has passed through such low-quality sampling no longer has the original frequencies. Once you sample audio, you can only throw bits away, you can't restore them.