Yes, you can significantly improve the sound even while keeping the factory navigation system.

By adding sound-tuning equipment, speakers, and a power amplifier—while continuing to use your current navigation or display audio—you can greatly enhance listenability and the sense of presence.
Since the appearance and operation are less likely to change, this is also a great option for those who want to keep the convenience of the factory setup.

For example,

• You want vocals to be clearer.
• You want to reduce the feeling that the sound is muffled.
• You want the bass to sound a bit more comfortable/pleasant.
• You want a sound that won’t be harsh even when you turn the volume up.

These kinds of concerns can often be addressed even with the factory navigation system.

What you can do will vary depending on your vehicle and the current condition of your audio, but the main options are typically the following.

• Speaker replacement
  Since it changes the “sound outlet,” it’s an approach where you can often feel the difference.

• Adding sound-tuning equipment
  You can fine-tune the audio balance so it’s easier to listen to within the cabin.

• Adding a power amplifier
  The sound gains more headroom, making the overall audio clearer and easier to hear.

• Adding a subwoofer (bass speaker)
  It’s effective when you want to add impact.

Even for sources you often use—such as radio, Bluetooth, USB, TV, and navigation voice—there are many cases where improvements can be made to match the factory system.
A major benefit is that you can aim for better sound quality without dramatically changing how you normally use it.

However, there are also some points to keep in mind.
Depending on your vehicle and the specifications of the factory equipment, the range of what can be done will differ.
Depending on the vehicle model, installation may require some ingenuity, and depending on what you want to achieve, it may be better to adjust the combination of components.

Also, compared with replacing the factory navigation unit itself, there may be differences in how much adjustment you can make.
That said, it’s absolutely possible to produce sound that satisfies you well while still maintaining everyday comfort.

This is a very compatible method for anyone who wants to improve sound quality while keeping the factory look and functions as-is.
You can start from a reasonable place that fits your budget and preferences.