Improve the quality of your audio
Director of Technology
Dynamic Captioning, LLC
First, I'm excited to talk about audio, because that's actually what I got my degree in (even though I now make software and products to help the deaf).
There is a lot you can do to help "muffled" audio. First, make sure you have enough of an amplifier to drive your headset / headphones. Headsets that plug into the phone directly are notoriously underpowered. They had to be, as most phones draw their power directly from the phone line and the phone company has limits on how much power they can draw. Even with new services like Vonage and MagicJack, where you have your own router with its own power supply, you have a phone that has to comply with the power limits set by the FCC. That is why the Gentner / amplifier approach helps so much: You have an external amplifier that has some "oomph" to it.
Secondly, phone companies have been reducing the bandwidth for telephone calls for some time, now. Their data compression algorithms are optimized for human speech without background noise. When you get a full broadcast program's background, music, and the "deep-voiced announcer" in the mix, those algorithms start to break down quickly. You can combat this with a practice known as "equalization." That is, to boost certain audio frequencies and lower others. In the past, this was expensive to get, as you had to have either an outboard equalizer or an audio mixer with a built-in equalizer.
Nowadays, though, equalization is not too expensive, at all. You can get an Alesis S-6 audio mixer with a 3-channel equalizer on their audio channels for about $70 at MusiciansFriend.com
Use this mixer in place of the headphone amplifier, and you can take the "low" down some, and boost the "mid," and you will get noticeably more understandable speech from your program. Given that it has multiple inputs, you can hook your telephone coupler into one input, and your computer output into another, and you don't even have to rewire to switch from landline to Skype/Google Voice. Your existing cabling for your Gentner should plug right into the mixer, but you'll probably need a 1/8" stereo to RCA adapter cable for your computer ... and make sure your headphones can plug into a 1/4" headphone jack. If you have a 1/8" headphone plug, you can get an adapter, but it's likely you want better headphones if you're using a 1/8" plug.