DIAGNOSIS The single small speaker can easily get overwhelmed and distort. It's also possible something is physically blocking the rear speaker grill or a software equalizer setting is off.
FIX First, check the physical device. Is it pushed flush against a wall or is something (like a cable) covering the speaker grill on the back? Give it a few inches of clearance. Next, in the Alexa app, go to Device Settings > Sounds and adjust the Bass/Mid/Treble sliders. Try reducing the Treble a notch.
IF THAT DIDN'T WORK Go to the same Sounds menu and turn off "Adaptive Sound." This feature tries to adjust EQ based on room noise and can sometimes make things sound worse in certain environments.
HEADS UP The Echo Show 5 just isn't a powerhouse for audio. If you're playing music loud, it will distort. For better sound, pair it with a Bluetooth speaker in the device settings.