Low volume, strange sound or hum in the audio output – these are symptoms of aged or leaked electrolyte capacitors.
Electrolyte from capacitors can cause severe damage, sometimes unrecoverable. Electrolyte causes corrosion of copper traces. Result can be completely eaten traces on printed circuit board, released components, etc. In this case, repair is very difficult or almost impossible. I’ve seen several expensive players completely destroyed by capacitor electrolyte.
If there is sigh of faulty electrolyte capacitors, they must be replaced immediately!

Do I always need to replace capacitors?
Some players are known to use capacitors that never leak (Sony DD models with center gear mechanics), while other are known to have always leaked capacitors (like cheaper Sony DD models with plastic mechanics – WM-DD10, WM-DD11, then most Aiwa players, early Panasonic models, etc.)
Those capacitors that survived 30 years, will probably work next several decades. Modern capacitors have better specification, but usually, like modern electronics of nowadays, will not last long.

In my opinion, replacing all capacitors is not good idea. Replacing capacitors because of sound improvement requires very high level of knowledge of electronics circuit design. In most cases, even using better quality capacitors does not automatically mean better sound quality. In some situations, it can make things even worse. And basic question is: Do you really want your device sound perfect and sterile? If yes, connect headphones to your smartphone and that’s it. We do not live in sterile environment, there is lot of distortion everywhere around us. That’s why, for human being it is more pleasant to listen to analog music than perfectly clean digital sounds. Simply, when you like sound of your player, leave it as it is. If not, try to improve it, perhaps by changing capacitors, mechanical setup or getting better player.

What to use as replacement?
It depends on where the capacitors are located. I recommend to use Tantalum-Polymer capacitors wherever possible
– Tantalum-Polymer capacitors have very low ESR, lower than electrolyte capacitors. Also linearity is good.
– They are very small, usually produced for small voltage. It is not problem in walkmans
– They are easy to solder
– They do not leak
– They’re more expensive than standard capacitors, but it is no tragedy

How to replace capacitors?
Answer is simple. Solder skills is required.

– I use hot-air soldering station to remove capacitors
– Then I clean PCB with IPA and cotton wool. It is very important to clean t well!
– Pads must be clean, apply new solder tin to their surface before you install new capacitor