Doubt in being mindful when two tasks are going to happen? When we are doing some work(eating food--first task).. we hear something important  or some noise(second task), which might require us to pay attention to it(to second task)..

(means if we want to be truly mindful, then how we have to deal with this situation)

NOTE: In this case, the first task or work needs continuous attention stream of mindfulness from us..
and second one tries to hinder or abrupt first task's flow..
What you are seeing or describing here is the impermanent and uncontrollable nature of conditioned phenomena.
The ultimate solution is to win Nibbana, the unconditioned. Here there exists no arising or passing away.
While in Samsara, you can practice Vipassana meditation and change meditation object according to whatever arises. You don't have to note everything. Try to balance the faculties to ensure restless and agitation does not arise.
Eat and be mindful of the entire process and if/when another object comes into contact with a sense door, e.g. the ear-door, note that object and return then to your primary meditation object, i.e. the eating-process.
The below Instructions for Mindful Eating is given by Ven. Mahasi Sayadaw. Note that these detailed instructions are used on retreat or when you have enough free time to practice in this way. If not, then use general awareness of the eating-process.

Instructions for Mindful Eating
1.Look at the food, noting &quot;seeing.&quot;
2.Notice whether or not you are hungry.
3.Note &quot;intending to move.&quot;
4.Turn your right or left hand laterally until it's resting on edge, perpendicular to the knee. Stop.
5.Raise your hand straight up until it's more or less at table-height. Stop.
6.Move your hand forward toward the fork or spoon. Stop.
7.Grasp the fork, being aware of the sensation of touch as you do so. Stop.
8.Lift the fork. Stop.
9.Move the fork toward the food. Stop.
10.Place a bite of food on the fork. Stop.
11.Slowly raise the fork to your mouth. Stop.
12.Touch the fork to your lips. Stop.
13.Open your mouth. Stop.
14.Put the food in your mouth. Notice the feel of your lips touching the fork, and the contact or temperature of the food on the tongue. Don't let yourself chew yet. Stop.
15.Lower your hand slowly. Stop.
16.Place the fork on your plate. Stop.
17.Return your hand to your knee. (You still haven't begun to chew.)
18.Chew the food. Flavor will appear. Mentally try to isolate the flavor from the tactile mass of the food, the movement of your mouth, and any desire that arises. Flavor is its own object, separate from all these things. If the flavor is strong you should focus on it. But if the flavor is bland, focus on the movement of the jaw or tongue. Note &quot;moving,&quot; &quot;tasting,&quot; &quot;desire,&quot; &quot;touch,&quot; and so on, as appropriate.
19.As you swallow, note &quot;swallowing.&quot;
20.Be aware your mouth is empty.
21.Notice whether or not you feel full.
22.Note the intention to take another bite.
23.Repeat steps 1-22. (Although mental notes were not given for all the steps, you may wish to add them, labeling &quot;moving&quot; for steps 3-5 and so on.)
If you need both hands to cut something, focus your attention on one hand only. Apply the same step-by-step procedure to drinking: observe the intention first; extend your hand; grasp the cup; move the cup toward you; take a sip of water; and finally, bring the cup back to the table, stopping completely after each action.
When mindfulness and insight are strong you may not recognize what you are eating. This experience is a revelation. Instead of green beans or rice on the plate, you see only color. Rather than meat or cheese, you taste an unnamed burst of sensation. This experience is difficult to describe, but if it happens you will immediately understand what we are talking about.
The step-by-step eating technique is intended mainly for meditation retreats. During daily life it usually isn't practical to eat this way, except on special days or when you have free time on the weekends. When you are unable to eat in the step-by-step manner just use general mindfulness to be aware of what you are doing as you eat the meal.