How to assign fixed ip to eth0 adapter in pi zero running 2019 image?

Using your 2019 image on pi zero 1.3 without wifi but connected to Internet with usb eth0 adapter. Can access the web menu but I am unable to get fixed IP assigned to the eth0. Any suggestions?
Also at some volume level, the sample sound becomes stuttered, how can we eliminate that?
Sometime a key on the keyboard seems to get stuck, pressing it again releases the key sound. How can I get rid of the key stucks which are annoying during live performances.

When plugging the SD in a PC: /boot is the root of the first (or only) partition of the inserted/mounted SD, so then read "/boot/samplerbox" as "drive: /samplerbox"

The /boot/samplerbox/wifistatips.txt is actually the /etc/dhcpd.conf.
Standard linux, so: in there you need an entry for interface eth0 like the wlan0, but without the "ssid" line. Adapt addresses to your situation.
This instructs dhcpd to use the info in there instead of trying to obtain the address from the network.

For volume issues Hans supplied a useful explanation on the more info page.

In my experience, missing note-offs quite aften arise from midi routing or cheap midi interfaces. So in general inspect your setup.
 ...but...
As you're using PI-zero with an image obtained here, it may even be a performance issue. All the nice extra's require resources.